Question 1019531
f, father now
s, son now


{{{system(f-20=s+10,f+s=50)}}}


{{{f=s+10+20}}}
{{{f=s+30}}}
-
substitute:
{{{(s+30)+s=50}}}
{{{2s+30=50}}}
{{{s+15=25}}}
{{{highlight(s=10)}}}