Set up the equation:

40 represents the money Betty had initially. 12 is the coefficient for x, and represents how many bags of popcorn Betty bought. x represents the cost of a single bag of popcorn. 10 is the money Betty is left with.
Subtract 40 from both sides:

Divide both sides by -12 to get x by itself:

The cost of a single bag of popcorn is
$2.50.