Number System


  1. How many 3-digit numbers, in all, are divisible by 6 ?









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    First 3–digit number divisible by 6 = 102
    Last such 3-digit number =996
    ∴ 996 = 102 + (n –1) 6
    ⇒  (n – 1)6 = 996 – 102 = 894

    Correct Option: B

    First 3–digit number divisible by 6 = 102
    Last such 3-digit number =996
    ∴ 996 = 102 + (n –1) 6
    ⇒  (n – 1)6 = 996 – 102 = 894

    ⇒ n – 1 =
    894
    = 149
    6

    ⇒  n = 150
    Hence , required answer is 150.


  1. The total number of integers between 100 and 200, which are divisible by both 9 and 6, is :









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    A number is divisible by 9 and 6 both, if it is divisible by LCM of 9 and 6 i.e., 18.

    Correct Option: B

    A number is divisible by 9 and 6 both, if it is divisible by LCM of 9 and 6 i.e., 18. Hence, the numbers are 108, 126, 144, 162, 180, 198.
    Thus , the total number of integers between 100 and 200 are 6.



  1. The least number, which must be added to 6709 to make it exactly divisible by 9, is









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    A number is divisible by 9 if the sum of its digits is divisible by 9.
    Here, 6 + 7 + 0 + 9 = 22

    Correct Option: A

    A number is divisible by 9 if the sum of its digits is divisible by 9.
    Here, 6 + 7 + 0 + 9 = 22
    Now, 22 + 5 = 27, which is divisible by 9. Hence 5 must be added to 6709.


  1. If 5432*7 is divisible by 9, then the digit in place of * is :









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    A number is divisible by 9, if sum of its digits is divisible by 9.
    Let the number be p.
    ⇒  5 + 4 + 3 + 2 + p + 7

    Correct Option: C

    A number is divisible by 9, if sum of its digits is divisible by 9.
    Let the number be p.
    ⇒  5 + 4 + 3 + 2 + p + 7
    ⇒ 21 + p
    ⇒ 21 + 6 = 27, which is divisible by 9.
    ∴  p = 6
    Therefore , the digit in place of * is 6 .



  1. (49)15 – 1 is exactly divisible by :









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    As we know that ,
    xn – an is exactly divisible by (x – a) if n is odd.

    Correct Option: D

    As we know that ,
    xn – an is exactly divisible by (x – a) if n is odd.
    ∴ (49)15 – (1 )15 is exactly divisible by 49 – 1 = 48, that is a multiple of 8.
    Hence required answer is 8.