Question

Sally and Joan were able to dust the house in 3 hours together. It takes Joan 10 hours to finish the same job alone. Without help, how long would it take Sally to finish the same job ?

Answer 1

4.3 hours

Explanation:

Let the number of hours Sally does a job = x

Let the number did hours Joan does a job be equal to y

Sally and Joan were able to dust the house in 3 hours together.

1/x + 1/y = 1/3

It takes Joan 10 hours to finish the same job alone.

Hence,

1/x + 1/10 = 1/3

1/x = 1/3 - 1/10

Least common dey = 30

1/x = 10 - 3/30

1/x = 7/30

Cross Multiply

x × 7 = 30

7x = 30

x = 30/7

x = 4.2857142857 hours

Approximately = 4.3 hours

Without help, it would take Sally 4.3 hours to finish the same job.