The speed of a stream is 4 mph. If a boat travels 90 miles downstream in the same time that it takes to travel 45 miles upstream, what is the speed of the boat in still water?
A)14 mph
B)8 mph
C)12 mph
D)15 mph

Question

Quizplus · Accepted Answer

The speed of the boat in still water can be found by setting up an equation based on the distances traveled and the speeds. Let $x$ be the speed of the boat in still water. The speed downstream is $x + 4$ mph, and the speed upstream is $x - 4$ mph. The time to travel downstream and upstream can be expressed as $\frac{90}{x+4}$ and $\frac{45}{x-4}$, respectively. Since the times are equal, we have $\frac{90}{x+4} = \frac{45}{x-4}$. Solving this equation for $x$ gives $x = 12$ mph.

The Speed of a Stream Is 4 Mph