Question 897771
let’s Mary’s age be {{{M}}} and her brother Joey {{{J}}}
given

Mary is 17 years old and her brother Joey is 5 years old. 

means {{{M=17}}} and {{{J=5}}}
In how many years, let’s say {{{x}}}, will Mary be just twice as old as Joey?

means {{{M +x=2(J+x)}}}.... plug in given values 

{{{17 +x=2(5+x)}}}...solve for {{{x}}}

{{{17 +x=10+2x}}}

{{{17 -10=2x-x}}}

{{{7 =x}}}....so, in {{{7}}} years Mary will be {{{M=17+7=24}}} and {{{J=5+7=12}}}

as you can see, {{{24=2*12}}}