Being able to find the change from a $10 bill is a valuable skill. You simply start with the purchase price ad count up to the next multiple of 10, then count up by 10's to the next dollar, and then count up to 10 dollars.
Purchase: $7.40 This amount is already a multiple of 10, so we count up to the next dollar (60 cents) which is 8 dollars, then to 10 dollars (2 dollars). The total of 60 cents + 2 dollars is $2.60. Easy!
Purchase: $4.65 Count up to $4.70 (5 cents), then to $5 (30 cents) and then to $10 ($5). The total of 5 cents + 30 cents + $5 = $5.35. Easy!
Strive to be able to easily find the change from a $10 bill, then strive to increase your speed. Good luck and enjoy the challenge!