The answer is 6
hope this helps!
So, let's make these two values improper fractions so they're easier to work with. 
So, to get the rate he grew in those months we have:
And to get the rate he grew in one month we have to divide both our numerator and our denominator by 21 to get:
aka he grew 1.24 inches in a month.
Answer:
x=1
Step-by-step explanation:
This stuff is quite easy in reality. Here's a little example including your numbers.

by this point we are left with simplifying it. Since it's a double negative we get a positive. Thus x=1
the answer is x= 10.
the steps would go like this.
4 + 3/5 × (15 + 2x) = 25
4 + 9 + 6/5x = 25
13 + 6/5x = 25
65 + 6x = 125
6x = 125 -65
6x = 60
x = 10
hope this helps