there are 25 dimes in $2.50. And there are 60 minutes in and hour. if he dropped 2 dime every 5 minutes. And 60 divided by 5 is 12. so he would have dropped 2 dimes 6 times. So then you have to take 2x6 which is 12. then you would take 10x12=120. then 250-120=130 and then insert the decimal giving you $1.30. So he had $1.30 left. Hope this helped