in the days when an ice cream sunda

in the days when an ice cream sundae cost less, a 10 years old boy entered a hotel coffee shop and sat at a table. A waitress put a glass of water in front of him.
How much is an ice cream sundae?
50 cents, replied the waitress.
the little boy pulled his hand out of his pocket and studied a number of coins in it. How much is a dish of plain ice cream? he inquired. some people were now waiting for a table and the waitress was a bit impatient. 35 cents, she said brusquely.
the little boy again counted the coins. I'll have the plain ice cream, he said. the waitress brought the ice cream, put the bill on the table and walk away. The boy ate it, paid the cashier and departed
when the waitress returned, she began wiping down the table and then swallowed hard at what she saw. there, place neatly beside the empty dish, were 15 cents- her tip