If they work together. Mark, Naomi, and Yvonne can sort some

Library books in 11 hours. Working alone, it would take Yvonne

5 hours and Mark 6 hours to do the same job. How long to the

nearest quarter hour) would it take Naomi, if she worked alone?

It will take Naomi 22 hours

Step-by-step explanation:

If they work together Mark, Naomi, and Yvonne take 11 hours, which means

three people represent 3 * 11 = 33 man * hours

If they work alone Yvonne will take 5 man hours, and Mark will take 6 * man hours.

Now we know that together they take 33 man hours therefore we have

5 + 6 + N = 33

Where N is Naomi's man * hours. We solve for N and get:

N = 33 - 11 = 22

N = 22

Naomi will take 22 hours to finish the job if she worked alone.