A boy swims downstream 09 miles in the same amount of time t

A boy swims downstream 0.9 miles in the same amount of time that he can swim 0.6 miles upstream. If the rate of the current is 1.5 mph, how fast can he swim in still water?

D=RT OR T=D/R
T=.9/(R+1.5)
T=.6/(R-1.5)
.9/(R+1.5)=.6/(R-1.5)
.6(R+1.5)=.9(R-1.5)
.6R+.9=.9R-1.35
.6R-.9R=-1.35-.9
-.3R=-2.25
R=-22.5/-.3
R=7.5 MPH IS HIS QWIMMING SPEED IN STILL WATER.
PROOF:
.9/(7.5+1.5)=.6/(7.5-1.5)
.9/9=.6/6
.1=.1