Story Problems: Make Them Real

Stacey Koprince —  September 16, 2013 — 2 Comments

gmat story problemsI’ve been on a story problem kick lately. People have a love / hate relationship with these. On the one hand, it’s a story! It should be easier than “pure” math! We should be able to figure it out!

On the other hand, we have to figure out what they’re talking about, and then we have to translate the words into math, and then we have to come up with an approach. That’s where story problems start to go off the rails.

You know what I mean, right? Those ones where you think it’ll be fine, and then you’re about 2 minutes in and you realize that everything you’ve written down so far doesn’t make sense, but you’re sure that you can set it up, so you try again, and you get an answer but it’s not in the answer choices, and now you’re at 3.5 minutes or so… argh!

So let’s talk about how to make story problems REAL. They’re no longer going to be abstract math problems. You’re riding Train X as it approaches Train Y. You’re the store manager figuring out how many hours to give Sue so that she’ll still make the same amount of money now that her hourly wage has gone up.

Try this GMATPrep® problem:

* ” Six machines, each working at the same constant rate, together can complete a certain job in 12 days. How many additional machines, each working at the same constant rate, will be needed to complete the job in 8 days?

“(A) 2

“(B) 3

“(C) 4

“(D) 6

“(E) 8”

Yuck. A work problem.

Except… here’s the cool thing. The vast majority of rate and work problems have awesome shortcuts. This is so true that, nowadays, if I look at a rate or work problem and the only solution idea I have is that old, annoying RTD (or RTW) chart… I’m probably going to skip the problem entirely. It’s not worth my time or mental energy.

This problem is no exception—in fact, this one is an amazing example of a complicated problem with a 20-second solution. Seriously—20 seconds!

You own a factory now (lucky you!). Your factory has 6 machines in it. At the beginning of the first day, you turn on all 6 machines and they start pumping out their widgets. After 12 continuous days of this, the machines have produced all of the widgets you need, so you turn them off again.

Let’s say that, on day 1, you turned them all on, but then you turned them off at the end of that day. What proportion of the job did your machines finish that day? They did 1/12 of the job.

Now, here’s a key turning point. Most people will then try to figure out how much work one machine does on one day. (Many people will even make the mistake of thinking that one machine does 1/12 of the job in one day.) But don’t go in that direction in the first place! If you were really the factory owner, you wouldn’t start writing equations at this point. You’d figure out what you need by testing some scenarios.

Six machines do the entire job in 12 days. I want to do the job in 8 days instead. First, how much of the job can my existing six machines do in 8 days? If those six machines worked just for 8 days, they would do 8/12 = 2/3 of the job. So I need to buy enough extra machines to do another 1/3 of the job during that 8 days.

Wait a second! If 6 machines can do 2/3 of the job in 8 days… then 3 machines can do half of that, or 1/3 of the job, in 8 days. That’s it! I’m done! That’s the 20-second solution. And it’s a solution I would never have found without simply thinking about how I’d figure this out in the real world, not on a standardized test.

The answer is (B): 3 machines.

Let’s try another GMATPrep problem.

* ” A used-car dealer sold one car at a profit of 25 percent of the dealer’s purchase price for that car and sold another car at a loss of 20 percent of the dealer’s purchase price for that car. If the dealer sold each car for $20,000, what was the dealer’s total profit or loss, in dollars, for the two transactions combined?

“(A) $1,000 profit

“(B) $2,000 profit

“(C) $1,000 loss

“(D) $2,000 loss

“(E) $3,334 loss”

 Okay, you know the drill: you’re the used-car dealer! One of your employees just came to you and told you that she sold a car for $20,000. You’re excited because you know that represents a 25% profit on what you paid for that particular car.

How much did you pay for it in the first place? Darn, you lost the record. If this were really happening, would you whip out a piece of paper and start writing equations? Of course not! You’d just try some numbers till you zeroed in on the answer. Try it yourself before you read the next paragraph.

Let’s see… was it $15,000? No, that would be a $5,000 profit, which is 33.3%. Is it a larger or smaller number? You want the profit to go down (it was only 25%) so you need the cost to be higher. How about $16,000? Let’s see, then the profit would be $4,000… and, bingo, $4k is indeed 25% of $16k! The first car gave you a $4,000 profit.

Okay, then another employee just told you that she finally got rid of that car that’s been sitting on the lot for ages. She was only able to get $20,000, though, even though you paid more for it.

This time, the car represents a 20% loss. How much did you pay for it? Try some numbers till you figure it out.

Let’s see. $22,000 would be a $2,000 loss… but $2k is nowhere near 20% of $22k. What about $25,000? That’d be a $5,000 loss. And, yes, $5k is 20% of $25k! Alright, so the second car cost you $25,000 and gave you a $5,000 loss.

Hmm. Not such a great day so far, huh? The first car was a $4,000 gain but the second was a $5,000 loss. That’s a net loss of $1,000.

The answer is (C): $1,000 loss.

The beauty of this “make it real” method is two-fold. First, when you can do the problem, it’s a whole lot easier to do it “logically” rather than in the “textbook math” fashion. Second, it’ll be much more obvious when you can’t do the problem, so it will be easier to let go and move to the next problem.

Key Takeaways for Story Problems

(1) Whenever you read a story, ask yourself what you would do if you had to figure this out in the real world. 99% of the time, you’d never think of writing equations. Instead, you’d “logic” it out. Where will the train be after 1 hour? 2 hours? If you increase Sue’s wage by 10%, what will happen? 20%?

(2) This will feel slow and funny at first, because you’re not used to treating standardized tests this way. Also, you may need to develop something that math teachers call “numbers sense” or “math sense.” Math sense is the ability to do the kind of back-of-the-envelope thinking that was demonstrated in this article. In a lovely bit of symmetry, this skill will be really useful in b-school and in your business career.

(3) Practice this same kind of thinking in the real world, with real scenarios or made-up ones. What’s the easiest way to approximate that 18% tip that you want to leave? You and a friend are on different trains heading straight towards each other (on different tracks!). You want to wave to your friend when the two trains pass each other. When is that going to happen? (This last one is basically the same scenario depicted in OG Quant Supplement #119!)

 

* GMATPrep® questions courtesy of the Graduate Management Admissions Council. Usage of this question does not imply endorsement by GMAC.

Stacey Koprince

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Stacey Koprince is an Instructor and Trainer as well as the Director of Online Community for Manhattan Prep. She's also a management consultant who specializes in corporate strategy. She has been teaching various standardized tests for more than fifteen years and her entire teaching philosophy can be summed up in five words: teaching students how to think.

2 responses to Story Problems: Make Them Real

  1. Hi Stacey,

    For the second problem, might a suggest a quicker method, one akin to the method you suggested in the first problem?

    1st car – 25% profit, hence selling price is 1.25 times buy price. 1.25 * x = 20,000. x = 16,000
    2nd car – 20% loss, hence selling price is 0.8 times buy price. 0.8 * y = 20,000. y = 25,000
    Hence, 5000 loss and 4000 profit, translates to net 1000 loss.

    Avoids the trouble of guessing values, in my opinion. If someone is good with fractions compared to decimals, they can use 5/4 instead of 1.25 and 4/5 instead of 0.8

    Pranav

    • Yes, that is a great solution method! The article is meant to apply to people who aren’t as comfortable with the algebra – for someone who doesn’t just “get” how to set up the equation, they can still get there by thinking through the situation in real-world terms.

      I didn’t meant to imply that you shouldn’t use algebra… just that you don’t have to, particularly if the translation / equation stuff is giving you trouble. A high percentage of people (not including you!) struggle with the “textbook math” approach to these kinds of story problems.

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