it takes tom 15 minutes to was a car. it takes only 10 minutes with jerry helping. How long should it take jerry to wash the car by himself?

Question

it takes tom 15 minutes to was a car. it takes only 10 minutes with jerry helping. How long should it take jerry to wash the car by himself?

Answer 1

Tom washes a car in 15 minutes, so his washing rate is 1car/15minutes.

 

Now, Jerry's washing rate is 1 car/x minutes (we have to figure out what x is).

 

The two guys together can wash the car in 10 minutes, so:

 

1/15 + 1/x = 1/10

 

Multiply all terms by 30 (the LCD):

 

30 * 1/15 + 30 * 1/x = 30 * 1/10

 

=>

 

2 + 30/x = 3

 

=>

 

30/x = 1

 

Multiply both sides by x:

 

x * 30/x = x

 

=> 

 

30 = x; Jerry can wash the car by himself in 30 minutes.