There are several standard exercises which train students to convert percentages, decimals and fractions. Converting percentage to decimals for example is actually as simple as moving the decimal point two places to the left and losing the percent sign "%." Thus 89% is equal to 0.89. Expressed in fraction, that would be 89/100. When you drill kids to do this often enough, they learn to do conversion almost instinctively.
There are other sources for worksheets also. You can find many public schools and private schools which will provide free worksheets for you if you buy textbooks from the school.
In other words, they never memorized their multiplication tables! Many times I have seen a student do poorly due to a weakness in basic math facts they should have learned in third grade.
Solve vertical and horizontal computation problems