Joan decided to redecorate. She wasn't sure how many rolls of wallpaper she would need for the bedroom but she knew that Buffy next door had recently done the same job, and the two apartments were identical in size.

"Buffy," she said, "How many rolls of wallpaper did you buy for your bedroom?"

"Ten," said Buffy.

So Joan bought the ten rolls of paper and did the job, but had 2 rolls left over.

"Buffy," she said, "I bought ten rolls of wallpaper for the bedroom, but I've got 2 left over!"

"Yeah," said Buffy, "So did I."