Sean is 20 and his brother is 12. How many years ago was Sean three times as old as his brother?

Question

Please help 

Answer 1

Let S = Sean's age and let B = brother's age

Since Sean is 20 and the brother is 12 we know that S = B + 8

We want to know when was S = 3B

 

We have a system of simultaneous equations to solve by elimination or substitution

 

S = B + 8

S = 3B

 

Let's substitute 3B for S in the first equation

 

3B = B + 8

2B = 8

B = 4

 

So when the brother was age 4 Sean was three times his age.. so when S = 3(4) or S = 12 

 

Since Sean is now 20 and this incidence occurred when he was 12 that was 8 years ago