Now the question has described the formula of Body Mass Index (BMI) as:
BMI

Where 'w' is weight in pounds and 'h' is the height of the person in inches.
We need to find the weight of a 13 year old boy whose height is 60 inches and the BMI value of his body is 20.5.
Now, we have
,

Plugging the values in the formula of BMI we get:

Solving for 'w' we get:

Hence, the weight of 13 year old boy is 105 pounds.