16. A man can do a job in 15 days. His father takes 20 days and his son finishes it in 25 days. How long will they take to complete the job if they all work together?

A. Less than 6 days B. Exactly 6 days C. Approximately 6.4 days D. More than 10 days

Answer Answer: Option C Explanation: 1 day's work of the three persons = (1/15+ 1/20+1/25) = 47/300.
so, all the three together will complete the work in 300/47 = 6.4 days.

17. A machine P can print one lakh books in 8 hours, machine Q can print the same number of books in 10 hours while machine R can print them in 12 hours. All the machines are started at 9 a.m. while machine P is closed at 11 a.m and the remaining two machines complete the work. Approximately at what time will the work be finished?

A. 11:30 a.m. B. 12 noon C. 12.30 p.m. D. 1 p.m.

Answer Answer: Option D Explanation: (P+Q+R)'s 1 hour's work = (1/8+1/10+1/12) = 37/120.
Work done by P, Q and R in 2 hours = (37/120*2) = 37/60.
Remaining work = (1- 37/60) = 23/60.
(Q+R)'s 1 hour's work = (1/10+ 1/12) = 11/60.
Now, 11/60 work is done by Q and R in 1 hour.
So, 23/60 work will be done by Q and R in (60/11*23/60) = 23/11 hours = 2 hours.
So, the work will be finished approximately 2 hours after 11 a.m., i.e., around 1p.m.

18. A is twice as good a workman as B and together they finish a piece of work in 18 days.In how many days will A alone finish the work?

A. 24 days B. 22 days C. 27 days D. 21 days

Answer Answer: Option C Explanation: (A's 1 day's work) : (B's 1 day's work) = 2:1
(A+B)'s 1 day's work = 1/18
Divide 1/18 in the ratio 2:1
A's 1 day's work = (1/18 * 2/3) = 1/27
Hence A alone can finish the work in 27 days

19. A is twice as good a workman as B and together they finish a piece of work in 14 days. The number of days taken by A alone to finish the work is

A. 11 B. 28 C. 42 D. 21

Answer Answer: Option D Explanation: (A's 1 day's work) : (B's 1 day's work) = 2:1.
(A+B)'s 1 day's work = 1/14.
Divide 1/14 in the ratio 2:1.
therefore A's 1 day's work = (1/14*2/3) = 1/21.
Hence, A alone can finish the work in 21 days.

20. A is thrice as good a workman as B and therefore is able to finish a job in 60 days less than B. working together, they can do it in

A. 22 1/2 days B. 20 days C. 25 days D. 30 days

Answer Answer: Option A Explanation: Ratio of times taken by A and B = 1:3.
If different of time is 2 days, B takes 3 days.
If difference of time is 60 days, B takes (3/2*60) = 90 days.
so, A takes 30 days to do the work.
A's 1 day's work = 1/30, B's 1 day's work =1/90.
(A+B)'s 1 day's work = (1/30+ 1/90) = 4/90 = 2/45.
therefore A and B together can do the work in 45/2 = 22 1/2 days.