Answer and Explanation:
To answer this question, we can construct the ratio of types of flash cards:
new : old : total
5 : 10 : 15
We want to multiply all sides of the ratio by a number such that the total number of cards is 45. This number is (45 / 15) = 3.
↓ multiplying the ratio by 3
new : old : total
15 : 30 : 45
We can see that he studied 15 new flash cards today.