Question

A girls is twice as old as her sister. Four years hence, the product of their ages (in years) will be 160. Find their present ages.

Answer 1

Let the present age of girl be x years then, age of her sister `x/2` years

Then, 4 years later, age of girl (x + 4) years and her sister’s age be `(x/2+4)` years

Then according to question,

`(x+4)(x/2+4)=160`

(x + 4)(x + 8) = 160 × 2

x² + 8x + 4x + 32 = 320

x² + 12x + 32 - 320 = 0

x² + 12x - 288 = 0

x² - 12x + 24x - 288 = 0

x(x - 12) + 24(x - 12) = 0

(x - 12)(x + 24) = 0

So, either 

x - 12 = 0

x = 12

Or

x + 24 = 0

x = -24

But the age never be negative

Therefore, when x = 12 then

`x/2=12/2=6`

Hence, the present age of girl be 12 years and her sister’s age be 6 years.