EDIT: Here, I made you a capx, as requested. I was always lost with vague answers when I was new to construct 2 and capx helped me most.
I made you a yellow sprite with a instant variable, there you can set the time for each level. as soon as you touch the red sprite ( the goal), the remaining time will add to your coins.
https:[x]//www.dropbox.com/s/ih9wxal4f4c5h7a/TimeToCoints.capx
remove [x]