Question

A boy is 4 years od and his sister is three times as old as he is. when the boy is 12 years old, how old will his sister be?

Answer 1

When the boy is 4 years old, his sister is 3 times as old as he is. That his sister is 12 years old when the boy is 4 years old. When the boy is 12 years old (8 years later), we can see that his sister is $12+8=20\;years$ old.

Answer 2

His sister will be 20 years old when the boy is 12 years old.

Step-by-step explanation:

We are given that a boy is 4 years old and his sister is three times as old as he is.

This means that when the boy is 4 years old, then his sister will be ;  

        Sister's age  =  $4 \times 3$ = 12 years old

Now, it is given that when the boy is 12 years old, then how old will his sister be.

It means that when the boy is 12 years, it concludes that 8 years had been passed since he was 4 years old which means that there will be an increase of 8 years in the age of his sister.

So, now the sister's age when the boy is 12 years old = 12 + 8 = 20 years old

Hence, when the boy is 12 years old, his sister will be 20 years old.