Answer :

crisforp
25 - 21 = 4 questions of his history incorrectly ;
4 / 25 = 0.16 represents the fraction of problems he answered incorrectly ;
to calculate the percent he got correct, do 21/25 and get 0.84 or 84%, so the ones he got wrong would have to make up the rest of it so 1.00 or 100% minus 0.84 or 84% is equal to 0.16 or 16%

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