Leon has 4 boxes. Each box has 15 golf balls and 12 tennis balls.
Which expression shows the total number of balls Leon has in the boxes?
a. 4 × 12 × 4 × 15
b. 4 + 12 + 4 + 15
c. (4 × 15) × (4 × 12)
d. (4 × 15) + (4 × 12)