Question

Lena is 10 years old. She asks her dad how old he is. He tells her that the lowest common multiple of their ages is 14 times the highest common factor of their ages. She knows that her dad is at least 30 years old but younger than 45. Work out how old her dad is.

Answer 1

Answer:

  35

Step-by-step explanation:

We know the factors of Lena's age are 2 and 5. The least common multiple must have these factors and the factors of 14, so will at least have factors of 2, 5, and 7.

Apparently, the dad's age is 5·7 = 35.

___

The GCF is 5; the LCM is 70 = 5×14.

_____

Sometimes, I use a little 3-part diagram to think about LCM and GCF. Here, it would look like ...

  (2 [5) 7]

where the numbers in curved brackets (2·5) and the numbers in square brackets [5·7] are factors of the two numbers of concern (Lena's age, her dad's age). The middle number in both brackets [5) is the greatest common factor, and the product of all three numbers is their least common multiple.

Here, the product of outside numbers, 2·7 = 14, represents the ratio of the LCM to the GCF. We know that Lena's age has factors of only 2 and 5, so the numbers in the diagram have to be (2[5)7], where 2 and 7 are on the ends and 5 is in the middle.