Solve the missing part in the given problems in A by using the following formula:
A/P = R×B
R = A ÷ B × 100
B = A ÷ R
where:
A/P is the percentage or amount,
R is the rate and
B is the base

1. What is 20 % of 50?
2. 12 is 25 percent of what number?
3. What percent of 4 is 5?
4. If 15 % of a number is 40, what is the number?
5.4 percent of a number is 20. What is the number?​