Question 1073013
1.{{{A+B=20}}}
.
.
.
{{{600A+700B=660(A+B)}}}
{{{600A+700B=660(20)}}}
{{{600A+700B=13200}}}
2.{{{6A+7B=132}}}
Multiply eq. 1 by 6 and subtract from eq. 2,
{{{6A+7B-6A-6B=132-120}}}
{{{B=12}}}
Now use either equation to solve for A.