Answer:
27
Step-by-step explanation:
Let x represent the number of $0.15 stamps Ivan bought. Then the value of his stamps is ...
0.15x +0.40(35-x) = 7.25
-0.25x +14.00 = 7.25 . . . . . eliminate parentheses, collect terms
-0.25x = -6.75 . . . . . . . . . . . subtract 14.00
x = 27 . . . . . . . . . . . . . . . . . . divide by -0.25
Ivan bought 27 $0.15 stamps.