Let the number of customers served by first teller be x and number of customer served by 2 nd teller 200-x
If for first teller
1customer= 3 min
x=?
3x min
If for 2 nd teller
1 customer= 5 min
(200-x)=?
5(200-x)= 1000-5x
So
3x= 1000-5x
8x= 1000
x= 125 customers
Time taken
125×3= 375 minutes = 6hours 15 minutes