Question 324943
Let B be the sum of the boys' ages, G the sum of the girls' ages.
Let Ng be the number of girls, Nb the number of boys.
.
.
.
1.{{{(B+G)/(Ng+Nb)=18}}}
2.{{{B/Nb=20}}}
3.{{{G/Ng=15}}}
4.{{{Ng=20}}}
From eq. 3 and 4, 
{{{G=300}}}
From eq. 2, 
{{{B=20Nb}}}
Substitute into eq. 1,
{{{(20Nb+300)/(20+Nb)=18}}}
{{{20Nb+300=18(20+Nb)}}}
{{{20Nb+300=360+18Nb}}}
{{{2Nb=60}}}
{{{highlight(Nb=30)}}}