Question 25940
Here's one approach: Let the tellers be Teller A and Teller B.
If A can do the job in 20 minutes, then he/she can do 1/20 of the job in 1 minute.
If A and B together can do the job in 15 minutes, then they can do 1/15 of the job in 1 minute.

So, if we subtract A's part of the job for 1 minute from the part of the job for 1 minute for A + B, then we will have the part of the job done by B in 1 minute.

A + B = 1/15
B = 1/15-A   But A = 1/20, so:
B = 1/15 - 1/20 
B = 4/60 - 3/60
B = 1/60 which means that Teller B can do 1/60 of the job in 1 minute, therefore, B can do the entire job in 60 minutes.

Answer: It would take Teller B 60 minutes to count the money.