Question

Lena is 10 years old. She asks her dad how old he is. He tells her that the lcm of their ages is 14 times the hcf 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

35

Explanation:

Let x represent the HCF of the ages. Then we can have ax = 10 represent Lena's age, and bx can represent her dad's age.

The LCM will be abx = 14x, so we have ab = 14.

Factors of 14 are 1·14 and 2·7. Since the ratio of ages is between 30/10 = 3 and 45/10 = 4.5, we must have b/a in that range. This will be the case for ...

a=2, b=7

For a=2 and ax=10, we have x=5.

The dad's age is bx = 7·5 = 35.