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