Solving word problems withD=R×T

Question

A fiaherman on pearl River rows 9 miles downstream in the same amount of time he rows 3 miles upstream. If the current is 6 mph, find how long it takes him to cover the 12 miles.

 

Answer 1

d = rt

t = d/r

 

rate downstream = boat speed + current

rate upstream = boat speed - current

 

t = 9/(b+6)

t = 3/(b-6)

 

Since the time upstream is same as time downstream

set the equations equal.  Use this to find the time for either leg of the trip.  Doubling that gives the total time.

 

9/(b+6) = 3/(b-6)

cross multiply

9(b-6) = 3(b+6)

9b-54=3b+18

9b-3b = 54+18

6b = 72

b = 12

 

t = 3/(b-6) = 3/(12-6) = 3/6 = 1/2

 

One leg of the trip took a half hour.

The round trip took 1 hour.