Buying Ping Pong Balls

Alice bought a number of Ping-Pong balls in a store where a 5 percent sales tax is added to every purchase. If she did not have to pay tax, she could have bought three more balls for the same amount of money. How many balls did she buy?

Extension:
If she did not have to pay tax, she could have bought w more balls for the same amount of money. State the percent sales tax in terms of w.

(Source: Mathematics Teaching in the Middle School, Mar-Apr 1996)