The answer to this would be
b=7/v
When multiplying numbers with exponents, you have to add the exponents. So 4r^3×2r^2 equals 8r^5.
In order to match each of the pairs you need to divide the numerator by the denominator to make it into a decimal.
Below are the pairs:
21/25 = 84%
13/20 = 65%
2/5 = 40%
3/4 = 75%
3/5 = 60%