Least number with a fixed remainder: Find the smallest number that leaves remainder 2 when divided by each of 12, 15, 20, and 24.
Aptitude
Problems on H.C.F and L.C.M
Difficulty: Easy
Choose an option
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A122
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B120
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C124
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D126
Answer
Correct Answer: 122
Explanation
Introduction / Context:Numbers leaving the same remainder upon division by several divisors are handled using the Least Common Multiple (LCM). The pattern is N ≡ r (mod each divisor), hence N − r is a common multiple of all divisors.
Given Data / Assumptions:
- Divisors: 12, 15, 20, 24
- Common remainder: r = 2
- We seek the least positive N such that N ≡ 2 for all divisors.
Concept / Approach:Compute LCM of the divisors, then set N = LCM + r for the least positive solution. In general, all solutions are N = LCM * k + r, k ≥ 1 integer, but k = 1 gives the smallest positive answer beyond r itself (which will not satisfy divisibility here).
Step-by-Step Solution:
Prime factors: 12 = 2^2 * 3; 15 = 3 * 5; 20 = 2^2 * 5; 24 = 2^3 * 3.LCM = 2^3 * 3 * 5 = 120.Least N = 120 + 2 = 122.Verification / Alternative check:
122 ÷ 12 leaves 2; 122 ÷ 15 leaves 2; similarly for 20 and 24, remainder is 2 each time.Why Other Options Are Wrong:
- 120 is divisible by the moduli (remainder 0, not 2).
- 124, 126 do not leave remainder 2 with all the given divisors simultaneously.
Common Pitfalls:
- Mistaking LCM itself as the answer rather than adding the fixed remainder.
Final Answer:
122