The Speed of a Stream Is $4 \mathrm{mph}$ If a Boat Travels 84 Miles Downstream in the a Boat

Multiple Choice

The speed of a stream is $4 \mathrm{mph}$ . If a boat travels 84 miles downstream in the same time that it takes to travel 42 miles upstream, what is the speed of the boat in still water?

A) $14 \mathrm{mph}$
B) $15 \mathrm{mph}$
C) $8 \mathrm{mph}$
D) $12 \mathrm{mph}$