With this question I do not understand the process or step of going from

a + b / b = 21

next step

a / b + 1 = 20

What rule is this? How does this come about?

answer a/b = 20

I think, the second step is a misprint:

should have been:

a/b + 1 = 21

Implies, a/b = 20

How (a+b)/b becomes a/b + 1 ? See below:

(a+b)/b can be written as a/b + b/b

cancelling b, we get a/b + 1

Hope this helps.

Scott,

The guide is correct as written. However, it is confusing how one step leads to the enxt.

The original statement reads a + b/b = 21

Looking at the left hand side of the equation, the denominator b actually applies to both a + b in the numerator:

a/b + b/b = 21

(Mathematically, if you had (1+2)/2, you could rewrite this as 1/2 + 2/2)

b/b = 1, so:

a/b + 1 = 21

a/b = 20

Hope that this is helpful! - Andrew