Question 904808
a+b=700,
5*a+3*b=2900
a=700-b
5*(700-b)+3*b=2900
3500-5b+3*b=2900
-2*b=-600
b=300
a=700-b
a=400 at $5 and  b=300 at $3
check
400+300=700
5*a+3*b=2900
5*400+3*300=2900
2000+900=2900
2900=2900
ok