Slow cashier works at - 1 Customer per 3 minutes.
Faster cashier works at - 1 Customer per 2 minutes.
So, every 6 minutes, both cashiers will serve exactly 5 customers.
(6 is the lowest common denominator for 3 and 2)
20 divided by 5 = 4.
4 x 6 = 24.
Between the both of them, it will take 24 minutes to serve 20 people.
In that 24 minutes, the slow cashier will serve 8 of the 20 customers.
and the faster cashier will serve the remaining 12.
