Answer:
3:4
Explanation:
We are given a situation where a student received a 75% on an exam. The exam was 20 questions long, then our question is to find the ratio of number of questions answered correctly and the total number of questions.
So let us break this up :
There are 20 questions total on the exam.
The student answered 75% of them correctly
Let us turn the amount he got correct into a fraction


Now multiply by 20 to find the amount the student got correct :


The student got 15 questions correct.
So our ratio is 15:20
Simplify :
3:4