+0

# Here's a question.

0
158
3
+5575

A motorboat goes 5 miles upstream in the same time it requires to go 7 miles downstream. If the river flows at​ 2mph, find the speed of the boat in still water.

hectictar  Sep 20, 2017
Sort:

#1
+79894
+2

A motorboat goes 5 miles upstream in the same time it requires to go 7 miles downstream. If the river flows at​ 2mph, find the speed of the boat in still water.

Distance  / Rate  = Time

Let the rate of the boat in still water  = R

Against the current the rate of the boat  = R - 2

With the current the rate of the boat  = R + 2

So......equate the time

5 / ( R - 2)   = 7 / ( R + 2)      cross-multiply

5 ( R + 2)   = 7 (R  - 2)     simplify

5R + 10  = 7R - 14      rearrange as

24   =  2R

12  = R    =    12 mph

The time it takes =   5 / [12- 2]  = 5 / 10   = 1/2 hr

CPhill  Sep 20, 2017
#2
+5575
+1

Ah...thank you!!!   Well now I see what was wrong...my equation was this:

5 / (R + 2)  =  7 / (R - 2)

hectictar  Sep 20, 2017
#3
+79894
+1

These are sometimes tricky...ones  !!!!

CPhill  Sep 20, 2017

### 7 Online Users

We use cookies to personalise content and ads, to provide social media features and to analyse our traffic. We also share information about your use of our site with our social media, advertising and analytics partners.  See details