Making Change

Change is what is left of the money you hand over. Ana buys a pen for 2.35 dollars and a notebook for 4.50 dollars, and pays with a 10 dollar note. Use c for change: c = 10 - 2.35 - 4.5. The board writes 4.50 as 4.5, since the zero at the end changes nothing.

Work left to right as always: 10 - 2.35 is 7.65, then 7.65 - 4.50 is 3.15, so c = 3.15. Ana gets 3 dollars and 15 cents back.

You could also add the two prices first, 2.35 + 4.50 is 6.85, and take that from 10. Both routes give 3.15, because taking away two prices one at a time is the same as taking away their total. Prices already have two decimal places, so nothing needs rounding.