The pin stall

Linh and Sam each made the same number of enamel pins for the school fair. Linh's pins are bigger, so she sells hers at 2 for $1. Sam sells his at 3 for $1.

At lunchtime Sam has to leave for football, so Linh sells both piles together at 5 for $2. Same thing, she reckons: 2 for $1 plus 3 for $1 makes 5 for $2.

When everything is sold, they have exactly $7 less than they would have made selling separately. They split the money half and half.