Jacob's printing inc. has two type of printing presses: model a and model b. model a can print 40 books per day and model b can print 70 books per day. altogether jacob has 12 printing presses. if he can ptint 600 books in a day then how many of each press does he have?

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

Hope i can you!