So you see...
The question in my math workbook is,

45 ÷ 3 = (30 + ___) ÷ 3
= 30 ÷ 3 + ___ ÷ 3
= 10 + 5
= 15

sorry my brain is broken right now- I have a feeling this problem is really easy to solve, but as I said my brain is broken lol