Based on the rate at which all three computers process bills, the time taken for the computers to process 1,780 bills is 4 minutes.
First find the number of bills left when the third computer joined:
= 1,780 - (160 x 1.5 mins) - (160 x 1.5 mins)
= 1,300
Then the average speed of all three computers is:
= 160 + 160 + 200
= 520 bills per minute
The time taken to finish up the other bills would be:
= 1,300 / 520
= 2.5 minutes
Add this to the minutes already elapsed before the third computer was used:
= 2.5 + 1.5
= 4 minutes
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