The simplest way to handle percentages is to convert to actualnumbers!

Assuming her 1990 long jump distance was "100" (just because that is easy to use percents on- the actual figure doesn't matter), 10% of 100 is 10 so her distance in 1991 was 110. 20% of the is 110*.2= 22 so in 1992 her distance as 110+ 22= 132. Now, 132/100= 1.32 or her distance in 1992 was 132% of her distance in 1990- 32% better.

(It's not 10+ 20= 30 because the second percentage was not "of" the same amount as the first.)