Working with fractions and percents
Adding or subtracting fractions needs a common denominator; multiplying does not, so just multiply across.
A worked example
For 2/5 + 1/6, rewrite both over 30: 12/30 + 5/30 = 17/30. For 4/7 x 3/8, multiply across to get 12/56, which is 3/14.
The mistake people actually make
Denominators are never added. And a 20% rise followed by a 20% fall does not return the original price, because the fall is taken from a larger amount.