Question: A password for a computer is six digits from 0 and 9. The six digit number is a prime number. What is the approximate probability that the last digit is a 3?
Options:
(A) 1/4
(B) 1/6
(C) 1/10
(D) 1/20
(E) 1/50
Ans: (A)
CoT: If the last digit is 0, 2, 4, 6, or 8, the number is even and therefore not prime. If the last digit is 5, the number is divisible by 5 and therefore not prime. The only possibilities are 1, 3, 7, and 9. So the probability is approximately 1/4 (answer A). We say approximately because there is a certain degree of randomness in the distribution of prime numbers.
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Question: 10 books are placed at random in a shelf. The probability that a pair of books will always be together is
Options:
(A) 1/10
(B) 9/10
(C) 1/5
(D) 3/10
(E) 1/2
Ans: (C)
CoT: 10 books can be rearranged in 10! ways consider the two books taken as a pair then number of favourable ways of getting these two books together is 9! 2!
Required probability = 1/5
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Question: A train running at the speed of 60 km/hr crosses a pole in 6 sec. What is the length of the train?
Options:
(A) 356
(B) 278
(C) 127
(D) 100
(E) 150
Ans: (D)
CoT: Speed = 60 * 5/18 = 50/3 m/sec
Length of the train = speed * time = 50/3 * 6 = 100 m
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