To make it easy, assume there were 100 questions in the original test, so 21 would be wrong.
When tested on the 21 incorrectly answered, 79% of 21=16.59 were answered correctly.
All together, 79+16.59=95.59 questions would be answered correctly, 95.59%.
Another way to solve: the total loss of marks is 21% of 21%=4.41% so that gives a score of 100-4.41=95.59%.