The speed of a stream is 3 mph A boat travels 4 miles upstre

The speed of a stream is 3 mph. A boat travels 4 miles upstream in the same time it takes it to travel 10 miles downstream.
What is the speed of the boat in still water?

D=RT
T=D/R
T=4/(R-3)
T=10/(R+3)
4/(R-3)=10/(R+3)
10(R-3)=4(R+3)
10R-30=4R+12
10R-4R=12+30
6R=42
R=42/6
R=7 MPH IS THE SPEED OF THE BOAT.
PROOF:
4/(7-4)=10/(7+3)
4/4=10/10
1=1