Give a quick guide: which percentage tool for which problem?

Question

Accepted Answer

Anchors for building any percent, swap to flip an awkward one, fractions for the "nice" values, and estimation when exact isn't needed.

Problem
Tool

15%, 35%, tips, discounts
10% / 5% / 1% building blocks

Ugly percent of a round number (8% of 25)
Swap: x% of y = y% of x

25%, 12.5%, 33%…
Fraction equivalents (1/4, 1/8, 1/3)

Reverse (part → whole)
part ÷ percent × 100

"Roughly how much?"
Rounding / front-end / Fermi

They all trade a little precision or setup for speed — choose by whether the task needs an exact figure or just a trustworthy ballpark.

Problem	Tool
15%, 35%, tips, discounts	10% / 5% / 1% building blocks
Ugly percent of a round number (8% of 25)	Swap: x% of y = y% of x
25%, 12.5%, 33%…	Fraction equivalents (1/4, 1/8, 1/3)
Reverse (part → whole)	part ÷ percent × 100
"Roughly how much?"	Rounding / front-end / Fermi