a=number of a models
b=number of b models
total number of books=abooks+bbooks
abooks=a times number of books per day or 40
bbooks=b times number of books per day or 70
total number of presses is 12 or a+b
so we have 2 equations
40a+70b=600 and
a+b=12
we can use eliminateion
multiply 2nd equation by -40 and add to top equation
40a+70b=600
-40a-40b=-480 +
0a+30b=120
30b=120
divide both sides by 30
b=4
sub back
a+b=12
a+4=12
minus 4
a=8
he has 8 of the A model machines and 4 of the B model machines
