In this question, we have to determine the % error of the estimate of the son. If he estimated the amount of money held in his dad's hand within 5% error, he can have all the coins. Otherwise, he will not have any.
The son's estimate is 81 cents, while the actual amount held in his dad's hand is 90 cents. The percentage error in the estimation is calculated by using the error and the actual value.
Error in the estimate = amount estimated - actual amount
= 81 cents - 90 cents = - 9 cents
Actual value = 90 cents
% error = (error/actual value) x 100 = (-9 cents / 90 cents) x 100
The boy has estimated 10% less amount than the actual value. Since his estimate is off by more than 5%, the son will not get any money from his dad.
Hope this helps.