Choose the number which is different from the others in the group.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 161
Note: The given answer is as per the official NTA Answer Key. However, the question has two possible answers, i.e. both Option (2) and Option (4), as explained below.
By the digits, 161 is the odd one out:
Option (1) 131 — digits 1, 3, 1, all odd.
Option (2) 161 — digits 1, 6, 1: the middle digit 6 is even, so 161 is the odd one out, the only number with an even digit.
Option (3) 151 — digits 1, 5, 1, all odd.
Option (4) 171 — digits 1, 7, 1, all odd.
By divisibility by 3 (a number is divisible by 3 when the sum of its digits is), 171 is the odd one out:
Option (1) 131 — digit sum 5, not divisible by 3.
Option (2) 161 — digit sum 8, not divisible by 3 (\(161=7\times23\)).
Option (3) 151 — digit sum 7, not divisible by 3.
Option (4) 171 — digit sum \(1+7+1=9\), so 171 is divisible by 3 (\(171=9\times19\)), the only one of the four.
Tip for students: When a question has two possible answers, mark the one you worked out yourself. If NTA’s answer key gives a different answer, challenge it when NTA opens the window for challenges.