Mathematics
Grade5
Easy

Question

The number that could NOT be a common denominator for these three fractions is?
1 third and 2 over 5 and 3 over 10.

  1. 15
  2. 30
  3. 60
  4. 90

hintHint:

The number which cannot be divided by Least Common denominator will be the answer of this question. We have to find the least common denominator and for that we have to find the least common multiple of denominators. Least Common Denominator (LCD) is the least common multiple of denominator of a set of fractions. The method of calculating the LCD is same as calculation of Least Common Multiple of fractions denominators. The least common multiple 

The correct answer is: 15


    The least common denominator is the lowest common multiple of the denominators of a set of fractions.
    Step1: We are given fractions 1 third and 2 over 5 and 3 over 10.
    And we have to find the LCM of denominators of the fractions i.e.,3 comma 5 comma 10
    Step2: We know that Multiples of 3 are:3 comma 6 comma 9 comma 12 comma 15 comma 18 comma 21 comma 24 comma 27 comma 30 comma 33 space horizontal ellipsis
    And the multiples of 5 are:5 comma 10 comma 15 comma 20 comma 25 comma 30 space horizontal ellipsis
    And the multiples of 10 are:10 comma space 20 comma space 30 comma space 40 comma 50 comma 60 space horizontal ellipsis
    Step3: Now we have to find the LCM of 3 comma 5 spaceand 10.
    From the above multiples of 3 comma 5 spaceand 10. it can be easily seen that the least common multiple of is 30
    Hence the least common denominator of  3 comma 5 spaceand 10 is 30.
    So any number which is not a multiple of 30 will be the answer of this question and from options it can be easily seen that 15 is not divisible by 30
    Multiples of 3 are 3, 6, 9, 12, …
    Multiples of 5 are 5, 10, 15, 20, …
    Multiples of 10 are 10, 20, 30, 40, …
    15 is not divisible by the denominator 10.

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