To find out how many sour fruit chews Keenan can buy, we need to divide the total amount of money he has by the cost of each sour fruit chew.
Keenan has $3.15, which is equal to 315 cents (since 1 dollar = 100 cents).
The cost of each sour fruit chew is $0.07, which is equal to 7 cents.
Now, we can calculate the number of sour fruit chews he can buy:
\[ \text{Number of sour fruit chews} = \frac{\text{Total amount in cents}}{\text{Cost per candy in cents}} = \frac{315 \text{ cents}}{7 \text{ cents}} = 45 \]
Therefore, Keenan can buy 45 sour fruit chews.