Smith can do a work in 15 days while Johnson can do it in 25 days. How long will they take if both work together? Choose the correct choice from the following:
A) 3 4/9 days  B) 8 4/9 days  C) 9 3/8 days  D) 6 3/8 days  E) 7 3/8 days
Explanation:   1/15 + 1/25 = 8/75   75/8 = 9 3/8 days   Answer: 9 3/8 days
<result>C</result>
Smith can do a work in 15 days while Johnson can do it in 25 days. How long will they take if both work together?