We use the simple facts that 10% is 1/10 of a number, and 1% is 1/100 of a number to practice finding percentages of numbers and quantities using mental math. This is a great technique and helps you with estimations in real life.
In the second part, I solve two simple word problems involving percentages, using mental math. One is about taxes and the other is about discounts.
How to find percent of a number using decimals — video lesson
Math Mammoth Grade 7 curriculum (pre-algebra)