Answer: there are 6 model A printing presses.
there are 3 model B printing presses.
Step-by-step explanation:
Let x represent the number of Model A printing presses.
Let y represent the number of Model B printing presses.
The company own 9 total printing presses. This means that
x + y = 9
Model A which can print 50 books per day and Model B which can print 30 books per day. The company own 9 total printing presses and this allows them to print 390 books per day. It means that
50x + 30y = 390 - - - - - - - - - - - -1
Substituting x = 9 - y into equation 1, it becomes
50(9 - y) + 30y = 390
450 - 50y + 30y = 390
- 50y + 30y = 390 - 450
- 20y = - 60
y = - 60/ - 20
y = 3
x = 9 - y = 9 - 3
x = 6