There were some pencils in a pencil box. Zoe added 7 more pencils to the pencil box. Now there are 24 pencils in the box. How many pencils were in the box to start with?

So if Zoe added 7 more pencils then all you have to do is just do the opposite

being the equation stated

X+7=24

X being the number of pencils before added

To find the answer just do the opposite

24-7=X

and X=17

This problem can be solved using subtraction. We want to know how many pencils were in the box before the 7 pencils were added, so we must subtract 7 from 24.

24 - 7 = 17.

There were 17 pencils in the box to start with.