math
posted by alicia on .
need help.
Young salmon hatch in freshwater steams and then migrate downstream to the ocean at the rate of 70 miles in 8 hours. there they live until they reach spawning age. then they return upstream to fresh water to spawn and usually die soon after. on the upstream trip, the salmon travel about 58 miles in 8 hours. find the rate of the salmon swimming in still water and the rate of the current.
this is what I have so far, but it doesnt add up
r*t=d
downstream= 8(x+y)=70
upstream=8(xy)=58

OK, it looks like you have chosen x for the rate in still water and y for the speed of the current. The equations you should have written are:
x + y = 70/8
x  y = 58/8
which is the same as you have written.
Adding them gives
2x = 128/8 = 16
x = 6 mph
y = 70/8  6 = 2.75 mph
You started the problem correctly and only needed to finish.