Free GRE Practice Question

A group of 50 visitors enters an exhibition. Each visitor buys either a $40 ticket or a $60 ticket, and at least one ticket of each price is bought. The average (arithmetic mean) price paid by the 50 visitors is greater than $50. Additional visitors, each buying a $40 ticket, are then to be added to the group so that the average price paid by the whole enlarged group becomes less than $50. Based only on the information given, which of the following could never be a number of additional $40 visitors large enough to make the average fall below $50?

Indicate all such numbers.