courtesy of user 'liahos1'

A store purchased 20 coats that each cost an equal amount and then sold each of the 20 coats at an equal price. What was the store's gross profit on the 20 coats?

(1) If the selling price had been twice as much the store's gross profit on the 20 coats would have been $2,400
(2) If the selling price had been $2 more, the store's gross profit on the 20 coats would have been $440

Answer is B (2) is sufficient

remember that gross profit = selling price - cost. since we don't know either of these, let's give them both variable names: s = selling price, c = cost.

we can rephrase the question as:
what is 20s - 20c?
we can divide this out by 20 to give, what is s - c?

NOTE: if you have ANY hesitation or uncertainty in generating equations like the ones i'm about to write below, then you should use an ORGANIZATIONAL CHART to solve this problem. in that chart, you should track the meaning of the different variables, as well as the relationships among them.
there are several different ways to make the chart, but it should focus on the idea that gross profit = selling price - cost, as mentioned above, as well as on the idea that total selling price = # of units x unit price.

(1)
this means that you change the selling price from s to 2s, while leaving c invariant.
20(2s) - 20c = 2400
divide by 20: 2s - c = 120
this doesn't allow us to find a value for s - c
insufficient

(2)
this means that you change the selling price from s to s + 2, while again leaving c invariant.
20(s + 2) - 20c = 440
20s + 40 - 20c = 440
20s - 20c = 400
s - c = 20
sufficient

answer = b