Learning how to find the change from a $1 bill is a useful skill. A $1 bill is worth 100 cents. You can simply count up to the next multiple of 10, then count by 10's to 100 cents.
Purchase: 34 cents Count up to 40 (6 cents) and from 40 to 100 (60 cents). 6 + 66 = 66 cents change
Purchase: 78 cents Count up to 80 (2 cents) and from 80 to 100 (20 cents). 2 + 20 = 22 cents change
Strive to be able to determine the change from a $1 bill, then strive to increase your speed. Good luck and enjoy the challenge!