Number System


  1. Which of the following numbers is prime ?









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    119 is divisible by 7, 187 is divisible by 11,247 is divisible by 13 and 551 is divisible by 19. So none of the given numbers is prime.

    Correct Option: D

    119 is divisible by 7, 187 is divisible by 11,247 is divisible by 13 and 551 is divisible by 19. So none of the given numbers is prime.


  1. The number (10n-1) is divisible by 11 for ?









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    For even values of n, the number (10n - 1 ) consists of even numbers of nines and hence it will be divisible by 11.

    Correct Option: C

    For even values of n, the number (10n - 1 ) consists of even numbers of nines and hence it will be divisible by 11.



  1. What least number must be added to 1056 get a number exactly divisible by 23 ?









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    On dividing 1056 by 23, we get 21 as remainder
    ∴ Required number to be added = (23 - 21 ) = 2.

    Correct Option: D

    On dividing 1056 by 23, we get 21 as remainder
    ∴ Required number to be added = (23 - 21 ) = 2.


  1. If in a long division sum,the dividend is 380606 and the successive remainders from the first to the last are 434, 125 and 413, then the divisor is ?









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    Let d = divisor and q = quotient

    First remove the last remainder:
    d x q + 413 = 380606
    d x q = 380193

    Correct Option: B

    Let d = divisor and q = quotient

    First remove the last remainder:
    d x q + 413 = 380606
    d x q = 380193

    So d cannot be even (choice d) or a multiple of 5 (choice c).

    If we try choice a or b,
    a quick division (by calculator) shows that
    380193 = 451 x 843, the other two choices.

    (In the absence of choices, we could find the prime factors of 380193,
    which are 3 x 11 x 41 x 281 and get various candidate divisors from that.)

    So one of them is the divisor and the other the quotient.

    So we can just check the first remainder:
    843 x 4 = 3372
    3806 - 3372 = 434 ... a match!

    And just to be sure:
    451 x 8 = 3608
    3806 - 3608 = 198

    So the divisor is 843.



  1. What least value must be given to * so that the number 6135 * 2 is exactly divisible by 9 ?









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    6 + 1 + 3 + 5 + x + 2 = 17 + x must be divisible by 9.
    So, x =1.

    Correct Option: B

    6 + 1 + 3 + 5 + x + 2 = 17 + x must be divisible by 9.
    So, x =1.