David and Terri drove a small motorboat down a river with a current. The rate the boat traveled in still water was r miles per hour, and the current's average speed was c miles per hour. It took them 1.5 hours to travel 4 miles downstream. Which of the following equations can be used to represent this information?
a. 1.5 = (r + c)4
b. 1.5 = (r - c)4
c. 4 = (r + c)1.5
d. 4 = (r - c) 1.5

Question

David and Terri drove a small motorboat down a river with a current. The rate the boat traveled in still water was r miles per hour, and the current's average speed was c miles per hour. It took them 1.5 hours to travel 4 miles downstream. Which of the following equations can be used to represent this information?
a. 1.5 = (r + c)4
b. 1.5 = (r - c)4 
c. 4 = (r + c)1.5
d. 4 = (r - c) 1.5