Percentages are how prices, marks, populations and growth are compared, and they sit underneath profit and loss, interest and data interpretation. Knowing common fractions as percentages and the successive-change formula lets you skip long division. Use it when a value rises or falls in steps, like a 25% hike followed by a 20% cut. The common trap is assuming "A is 20% more than B" means B is 20% less than A; the base changes, so it does not.