Answer:
1 month and a half
Step-by-step explanation:
You can convert 5/2 to 10/4 so now you have the same denominator. Now all you do is multiply it by the amount of months. After 2 months he will have grown 20/4 inches. You are trying to find 15/4 so you can see that if you want to get a more precise answer you can use 5/4 every half month.
That gives you 1 month and a half to get to 15/4 inches.
Hope this helps !!
<3