Tanika took a test that had 20 multiple- choice questions and 10 true/false questions. she got 9/10 of the multiple choice questions correct and she got 4/5 of the true/false
questions coorect.
how many multiple- choices questions did tanika get correct

This is a ratio based problem.
knowing that there are 20 mc questions and she got 9/10 of them right, we can solve the problem by getting the equivalent fraction that has the same denominator as the total number of mc questions ( in this case,20)

multiplying 9/10 by 2/2, we get 18/20 and that's the answer