Question 189681
Let s=Stevie's age and b=Brother's age


So this means that 

s-4 = Stevie's age 4 years ago, and

b-4=Brother's age 4 years ago



"Stevie is twice as old as his brother" translates to {{{s=2b}}}



"Four years ago he was four times as old as his brother" translates to {{{s-4=4(b-4)}}}



{{{s-4=4(b-4)}}} Start with the second equation.



{{{2b-4=4(b-4)}}} Plug in {{{s=2b}}}



{{{2b-4=4b-16}}} Distribute.



{{{2b=4b-16+4}}} Add {{{4}}} to both sides.



{{{2b-4b=-16+4}}} Subtract {{{4b}}} from both sides.



{{{-2b=-16+4}}} Combine like terms on the left side.



{{{-2b=-12}}} Combine like terms on the right side.



{{{b=(-12)/(-2)}}} Divide both sides by {{{-2}}} to isolate {{{b}}}.



{{{b=6}}} Reduce.



So Stevie's brother is currently 6 years old. Also, he was 2 years old 4 years ago.



{{{s=2b}}} Go back to the first equation



{{{s=2(6)}}} Plug in {{{b=6}}}



{{{s=12}}} Multiply



So Stevie is currently 12 years old. Four years ago, he was 8 years old. Notice how {{{8=2*4}}} (ie 8 yrs old is 4 times greater than 2 yrs old)