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 \(3*[\frac{1}{6}+\frac{1}{8}+r_{Mary}] = 1\)unit 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.