Answer:
18
Step-by-step explanation:
In both cases, the number remains constant, 240 stamps.
Given that 3:7 is equal to 3/10 to 7/10
Initially, Tom had 3/10*240=72 stamps
Finally, 3:5 equal to 3/8 to 5/8
Tom has 3/8*240=90 stamps
Difference is 90-72=18 stamps
Therefore, Tom bought 18 stamps from Adam and Adam didn't replace