Question 164479
An oil tanker can be filled in 10 hours from one pipe. It can be filled by another pipe in 8 hrs. How long will it take to fill the tank if both pipelines are used . 


Let x=amount of time it takes to fill the tanker when both pipelines are used
First pipe fills at the rate of 1/10 tanker per hour
Another pipe fills at the rate of 1/8 tanker per hour

Together they fill at the rate of 1/10 + 1/8=4/40 +5/40=9/40 tanker per hour.
So our equation to solve is
(9/40)*x=1 ( 1 tanker, that is) multiply each side by 40
9x=40 divide each side by 9
x=4 4/9 hours----------------------------answer

CK
First pipe:
(1/10)*(4 4/9)=(1/10)*(40/9)=4/9 of the tanker
Second pipe:
(1/8)*(40/9)=5/9
4/9 + 5/9=1
9/9=1
1=1

I THINK YOUR BOOK IS WRONG.

Hope this helps---ptaylor