Question 310882
{{{P}}}-Peter's age right now
{{{S}}}-Sister's age right now
Next year, Peter will be {{{P+1}}}, Sister will be {{{S+1}}}.
1.{{{P+1=2(S+1)}}}
2.{{{S=P-3}}}
Substitute eq. 2 into eq. 1,
{{{P+1=2(P-3+1)}}}
{{{P+1=2(P-2)}}}
{{{P+1=2P-4}}}
{{{-P=-5}}}
{{{highlight(P=5)}}}
Then from eq. 2,
{{{S=P-3=5-3}}}
{{{highlight_green(S=2)}}}
Peter is 5 years old, his sister is 2 years old.