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27 January, 22:51

Which pair of numbers is relatively prime?

a. 72 And 115

b. 120 And 835

c. 201 And 735

d. 303 And 1041

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Answers (1)
  1. 27 January, 23:04
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    Relatively prime means having no factors in common.

    I will start by checking if they have the factor "3" in common: it's easy to check: the sum of its digit needs to be divisible by 3:

    a. 72 And 115

    7+2=9

    1+1+5=7

    not true that both are divisible by 3

    b. 120 And 835

    1+2=3

    8+3+5=16

    not true that both are divisible by 3

    but, they're both divisible by 5! (0 or 5 at the end)

    not relatively prime

    c. 201 And 735

    2+1=3

    7+3+5=15

    both are divisible by 3!

    d. 303 And 1041

    3+3=6

    1+4+1=6

    both divisible by 3

    the answer is a)
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