Answer:
7
Step-by-step explanation:
If he ended up with 20 half pieces, then he cut 10 pieces in half since 20/2 = 10.
Before starting to cut he got 3 more pieces, so he had 3 fewer than 10.
Answer: He started with 7 pieces of paper.
Check:
Start with 7 pieces of paper.
Add 3 pieces of paper to end up with 10 pieces of paper.
Cut the 10 pieces of paper to end up with 20 half pieces.
He did end up with 20 half pieces, so he started with 7.
