Nathan deposit 7/9 of his allowance into a savings account he spent the remaining amount or $2.50

Nathan's total allowance is $11.25 ; amount deposited is $8.75

Given:
Savings = 7/9 of his allowance
Excess = $2.50 or 2/9 of his allowance

to get the amount deposited, we need to first get the total allowance. 

2.50 ÷ 2/9 ⇒ 2.50 * 9/2 = 22.5/2 = 11.25

Total allowance = 11.25
amount to be deposited: 11.25 * 7/9 = 78.75/9 = 8.75

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.