Question 147898
Joe's age is J.
Sally's age is S.
Joe is 5 years older than Sally.
1.{{{J=5+S}}}
In 15 years, Joe will be J+15.
In 15 years Joe will be 3 times as old as Sally is now.
2.{{{J+15=3S}}}
Substitute J from eq. 1 into eq. 2 to get an equation in S only,
2.{{{J+15=3S}}}
{{{(5+S)+15=3S}}}
{{{20+S=3S}}}
{{{20=2S}}}
{{{S=10}}}
From eq. 1,
1.{{{J=5+S}}}
{{{J=5+10}}}
{{{J=15}}}