aparnaharish wrote:

Ferman can do a job in 6 days and Kelly can do the same job in 8 days. They both undertake the job for $640. With the help of Mary, they finished it in 3 days. How much was paid to Mary?

A. $75

B. $80

C. $85

D. $100

E. $120

Rate of doing work for Ferman =

\frac{1}{6} and for Kelly =

\frac{1}{8}. Also, as

Time*Rate = Work,

we have

3*[\frac{1}{6}+\frac{1}{8}+r_{Mary}] = 1unit of work

Thus,

r_{Mary} = \frac{1}{3}-(\frac{1}{6}+\frac{1}{8}) \to r_{Mary} = \frac{1}{24}Thus, work done by Mary in 3 days :

3* \frac{1}{24} =

\frac{1}{8} units of work, and as the payment is directly proportional to the work done, the payment for her =

\frac{640}{8}= 80$

B.