Q: Three years ago, the average age of a family of 5 members was 17. A baby having been born, the average age of the family is the same today. What is the age of the child?
Options:
(A) 3
(B) 2
(C) 4
(D) 1
(E) 5
A: OK... Stream of consciousness: Present age of 5 members = 5 x 17 + 3 x 5 = 100 years
Also, present ages of 5 members + Age of the baby = 6 x 17 = 102 years
Age of the baby = 102 100 = 2 years.
The answer is (B).

Q: Pipes A and B can fill a cistern in 8 and 24 minutes respectively. They are opened an alternate minutes. Find how many minutes, the cistern shall be full?
Options:
(A) 10
(B) 26
(C) 12
(D) 24
(E) 48
A: OK... Stream of consciousness: 1/8 + 1/24 = 1/6
6 * 2 = 12
The answer is (C).

Q: Find the number which when added to itself 13 times, gives 112.
Options:
(A) 7
(B) 8
(C) 9
(D) 10
(E) 11
A: OK... Stream of consciousness: Let the number be x. Then, x + 13x = 112 14x = 112 x = 8.
The answer is (B).

Q: Find the average of the first 41 natural numbers?
Options:
(A) 8
(B) 9
(C) 10
(D) 11
(E) 21
A:
OK... Stream of consciousness: Average of the first 'n' natural numbers = (n + 1)/2
We get (41 + 1)/2 = 21
The answer is (E).