Number Theory Remainders Series

XAT 2015 · Quantitative Ability & Data Interpretation · Hard · Quantitative Ability

An ascending series of numbers satisfies the following conditions:

i. When divided by 3,4,53, 4, 5 or 66, the numbers leave a remainder of 22. ii. When divided by 1111, the numbers leave no remainder.

The 6th6^{\text{th}} number in this series will be:

  1. A.

    242

  2. B.

    2882

  3. C.

    3542

  4. D.

    4202

  5. E.

    None of the above

Answer

C

Explanation

LCM(3,4,5,6)=60(3, 4, 5, 6) = 60. Numbers leave remainder 2 when divided by 60, so N=60k+2N = 60k + 2. We need 60k+2≡0(mod11)  ⟹  5k+2≡0(mod11)  ⟹  5k≡9(mod11)  ⟹  k≡4(mod11)60k + 2 \equiv 0 \pmod{11} \implies 5k + 2 \equiv 0 \pmod{11} \implies 5k \equiv 9 \pmod{11} \implies k \equiv 4 \pmod{11}.

First value k=4  ⟹  N1=60(4)+2=242k = 4 \implies N_1 = 60(4) + 2 = 242. Subsequent values occur every 60×11=66060 \times 11 = 660. Nn=242+660(n−1)N_n = 242 + 660(n-1). For n=6n = 6: N6=242+660(5)=242+3300=3542N_6 = 242 + 660(5) = 242 + 3300 = 3542.

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