Question 963316
{{{A+B=21}}}
.
.
.
{{{A=1+B/3}}}
{{{3A=3+B}}}
{{{B=3A-3}}}
Substituting,
{{{A+3A-3=21}}}
{{{4A=24}}}
{{{A=6}}}
Then,
{{{6+B=21}}}
{{{B=15}}}