Percentages
A percent is a ratio out of 100; the core relationship is part = percent x whole, which can be solved for any one of the three quantities.
Common benchmark conversions worth memorizing: 1/8 = 12.5%, 1/4 = 25%, 1/3 is about 33.3%, 3/8 = 37.5%, and 2/3 is about 66.7%. They let you estimate answers before computing.