b - stands for before
a - stands for after
"4/7 as many cookies as cupcakes" (Before)
Cookie : cupcake
4units:7units
Lets call them 4b and 7b

"4/5 as many cookies as cupcakes" (after)
Cookie : cupcake
4unit:5unit
Lets call them 4a and 5a

For cookie, before - 12 = after so:
4before units - 12 = 4after units
(4b-12=4a)

For cupcakes, before - 31 = after so:
7before units - 31 = 5after units
(7b-31=5a)