Question 1066219
b, m, f
Boy, mother, father


{{{system(b=m/2,m=f-5,b+m+f=100)}}}


Substitute into the "100" equation all in terms of b.
{{{b+2b+(m+5)=100}}}
{{{b+2b+(2b+5)=100}}}
{{{5b+5=100}}}
{{{b+1=20}}}
{{{highlight(b=19)}}}