So how you can solve this is by using some cool tricks.
Look at the end number of computers, which is 40 right?
So with the total number of 40, you will simply subtract 10 and 5, to get the missing number.
Which is 40-10-5= 25
Then for printers you would do, 8-2= 6
Then by filling in the first blank you can move into the second blank by adding up all the three numbers you now have,
8+2+6= 16