Percentages

% turns a number into a percentage. Adding or subtracting a percentage scales the value; multiplying takes the percentage of it. off takes a percentage away, so 8% off 200 is 200 - 8%.

200 + 8%
200 - 8%
200 * 8%
8% of 200
8% off 200
(100 + 10%) + 10%

Two percentages combine as percentages. A percentage keeps a unit intact.

4% + 4%
20% * 20%
1 kg + 10%

A percentage divided by a number is still a percentage, which is what makes rate / 12 a monthly rate.

rate = 6.5%
rate / 12
(1 + rate) ^ 10