A shop sells one pound bags of peanuts for $2 and three pound bags of peanuts for $5. If 9 bags are purchased for a total of $36 how many one pound bags were purchases

X=number of 1lb bags

y=number of 3lb bags

2x+5y=36

and

x+y=9

mulitply 2nd equation by - 2 and add to first equation

2x+5y=36

-2x-2y=-18 +

0x+3y=18

3y=18

divide both sides by 3

y=6

sub back

x+y=9

x+6=9

x=3

there were 3 one pound bags bought