Answer:
3 inches
Explanation:
The brother is growing 3 inches per month, because if their 54 inches tall in January and 63 inches tall in April you do 63-54 to see how much he grown in three months which is 9. Then you do 9/3 and 3 is from their is a 3 month gap between January and April and you get 3 inches. So the brother grew 3 inches per month through January and April.