Posted by Madison on .
Mike can mow a lawn in 3 hours. Paul can mow the same lawn in 2 hours. How long would it take both of them working together to mow the lawn? The answer is suppose to be 20/9.
This is what I did and I get 9/20. What am i doing wrong?
1.4x+1/5x=1 which gives u 9/20. I'm so lost...

Math 
Reiny,
Mike's rate = 1/3 lawn/hr
Paul's rate = 1/2 lawn/h
combined rate = (1/3 + 1/2) lawn/hr
= 5/6 lawn/hr
time for combined rate = 1/(5/6) hours
= 6/5 hours
Their answer is wrong.
How can it take 20/9 or more than 2 hours for both of them to do it, when Paul alone could do it in 2 hours. 
Math 
Bob,
6/5hrs

Math 
Henry,
t = 3*2 / (3+2) = 6/5 hrs.