Since there are 60 questions in the test and she managed to answer 70% of the questions correctly, that means that she answered 70% out of 60 questions correctly.
So you need to calculate how much 70% out of 60 is. Since % is always measured on a base from 0 (%) to 100(%), then 70% is equal to the fraction 70/100, simplified to 7/10. To know how much 7/10 out of 60 is,you just multiplie them and your answer will be 42.