Reasoning: 1/20 - 1/40 = 1/40
40 minutes
Ans: (C)
Question: A pipe can fill a cistern in 20 minutes whereas the cistern when fill can be emptied by a leak in 40 minutes. When both pipes are opened, find when the cistern will be full?
Options:
(A) 17 minutes
(B) 87 minutes
(C) 40 minutes
(D) 27 minutes
(E) 11 minutes

Reasoning: There are 39 students and every one has equal chance of being selected as a leader. Hence, the leader can be appointed in 39 ways. When one person is appointed as leader, we are left with 35 students. Out of these 38 teachers, we can select one class representative. So, a class representative can be selected in 38 ways. Hence, the number of ways in which a leader and class representative can be selected = 39 x 38 = 1482
Ans: (D)
Question: From among the 39 students in a class, one leader and one class representative are to be appointed. In how many ways can this be done?
Options:
(A) 1360
(B) 1260
(C) 1060
(D) 1482
(E) 1460

Reasoning: A's 1 hour work = 1/4;
(B + C)'s 1 hour work = 1/2;
(A + C)'s 1 hour work = 1/2
(A + B + C)'s 1 hour work = (1/4 + 1/2) = 3/4
B's 1 hour work = (3/4 - 1/2) = 1/4
B alone will take 4 hours to do the work.
Ans: (C)
Question: A can do a piece of work in 4 hours; B and C together can do it in 2 hours, which A and C together can do it in 2 hours. How long will B alone take to do it?
Options:
(A) 2hours
(B) 3hours
(C) 4hours
(D) 5hours
(E) 6hours

Reasoning: Speed = 63 * 5/18 = 35/2 m/sec
Time taken = 280 * 2/35 = 16 sec
Ans: (B)
Question: A train 280 m long, running with a speed of 63 km/hr will pass a tree in?
Options:
(A) 22 sec
(B) 16 second
(C) 77 sec
(D) 55 sec
(E) 17 sec