The Speed of a Stream Is $6 \mathrm{mph}$ If a Boat Travels 54 Miles Downstream in the a Boat

Multiple Choice

The speed of a stream is $6 \mathrm{mph}$ . If a boat travels 54 miles downstream in the same time that it takes to travel 27 miles upstream, what is the speed of the boat in still water?

A) $18 \mathrm{mph}$
B) $20 \mathrm{mph}$
C) $12 \mathrm{mph}$
D) $21 \mathrm{mph}$