Micheal and Adam can do together a piece of work in 20 days. After they have worked together for 15 days Micheal stops and Adam completes the remaining work in 10 days. In how many days Micheal complete the work separately.  Choose the correct choice from the following answers
A) 40 days
B) 100 days
C) 120 days
D) 110 days
E) 90 days

Rationale: "rate of both = 1 / 20 together they do = 1 / 20 * 15 = 3 / 4 left work = 1 - 3 / 4 = 1 / 4 adam completes 1 / 4 work in 10 day so he took 10 * 4 = 40 days to complete the left work alone . thus the rate of adam is 40 / 1 rate of micheal = 1 / 20 - 1 / 40 = 1 / 40 thus micheal takes 40 days to complete the whole work . ans . a ."
Answer: A