Colin drove 30 miles to visit his sister. Then he drove 15 miles to a store. Later he plans to drive 26 miles to work. Which number sentence can be used to find how many miles he has driven so far?
a. 30 + 15 = _____
b. 30 + 26 = _____
c. 30 + 15 + 26 = ______
d. 30 - 15 = ______
a. 30 + 15 = _____
b. 30 + 26 = _____
c. 30 + 15 + 26 = ______
d. 30 - 15 = ______