7th Grade MATH: A 15% tip on a $14.95 lunch bill is about how much?

10% of $14.95 equals: $14.95 * 0.1 = $1.495. And half thereof are 5% = $0.7475.
15% = 10% + 5% = $1.495 + $0.7475 = $2.2425.

Well, as I am usually out of "millibucks", a sensible answer would be: $2.25.

But, actually, the ROUND-function for positive numbers to two(!) decimal places would say:

ROUND($2.2425, 2) := floor ($2.2425 * 10^2 + 0.5) / 10^2 = $2.24

The exponent 2 refers to the 2 decimal places, which is a sensible constraint in day-to-day business.
The floor-function simply determines the closest integer below a real number.