A and B have to type a book together containing 120 pages. A takes 9 hrs to type 36 pages and B takes 5 hrs to type 40 pages. A typed first 60 pages alone and the last 60 pages were typed by A and B together. How much time (in hours) will be taken to type the complete book?

A takes 9 hrs to type 36 pages

=> A's efficiency = $$\frac{36}{9}=4$$ pages/hr

Similarly, B's efficiency = $$\frac{40}{5}=8$$ pages/hr

A typed first 60 pages alone, => Time taken to print 60 pages = $$\frac{60}{4}=15$$ hours

Similarly, time taken to print last 60 pages by A and B together = $$\frac{60}{(4+8)}=5$$ hours

$$\therefore$$ Total time taken to complete the book = $$15+5=20$$ hours

=> Ans - (B)