Question 263127
D=RT OR T=D/R
TU=7/(R-C) TRAVEL UPSTREAM
TD=15/(R+C) TRAVEL DOWN STREAM.
THE TIMES ARE EQUAL.
7/(R-4)=15/(R+4)  CROSS MULTIPLY.
15(R-4)=7(R+4)
15R-60=7R+28
15R-7R=28+60
8R=88
R=88/8
R=11 IS THE SPEED OF THE BOAT IN STILL WATER.
PROOF:
7/(11-4)=15(11+4)
7/7=15/15
1=1