Question 152435
Distance(d) equals rate(r) times Time(t) or d=rt; r=d/t and t=d/r

Let r=rate(speed) of the current
(We know that speed upstream=15-r and speed downstream=15+r)

Time to go upstream=9/(15-r)
Time to go downstream=12/(15+r)

Now we are told that the above two times are equal, so our equation to solve is:

9/(15-r)=12/(15+r)  multiply each side by (15-r)(15+r) or cross multiply

9(15+r)=12(15-r)  get rid of paren
135+9r=180-12r  subtract 135 from and add 12r to each side

135-135+9r+12r=180-135-12r+12r  collect like terms

21r=45  divide each side by 21

r=2 1/7 mph---------------------speed of current

CK
9/(15-2 1/7)=12/(15+2 1/7) or
9/(12 6/7)=12/(17 1/7) and this equals
9/(90/7)=12/(120/7) or
63/90=84/120 and
7/10=7/10

Hope this helps----ptaylor