Subhash can copy 50 pages in 10 hours; Subhash and Prakash together can copy 300 pages in 40 hours. In how much time can Prakash copy 30 pages?

Answer: Option B

Number of page copied by (Subhash + Prakash) in 1 hour = $$\frac{300}{40}$$  = 7.5 pages;
Subhash copied pages in one hour = $$\frac{50}{10}$$ = 5 pages
Hence, Prakash copied pages in one hour = 7.5 - 5 = 2.5
Thus,
Prakash can copied 30 pages in = $$\frac{30}{2.5}$$ = 12 hour