1. what is 10+20%?

2. what is 10-20%?

The question can be misunderstood

10 + 20% = 10 + 20/100 = 10 + 0.2 = 10.2

But if the question is what is 10 increased by 20% it is

10 * (1 + 20%) = 10 * (1 + 0.2) = 10 * 1.2 = 12

So that is different.

Else with the other question

10 - 20% = 10 - 20/100 = 10 - 0.2 = 9.8

10 * (1 - 20%) = 10 * (1 - 0.2) = 10 * 0.8 = 8

10 - 20% = 10 - 20/100 = 10 - 0.2 = 9.8

10 * (1 - 20%) = 10 * (1 - 0.2) = 10 * 0.8 = 8