Set up the equation:
[tex]40 - 12x = 10[/tex]
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:
[tex]-12x = -30[/tex]
Divide both sides by -12 to get x by itself:
[tex]x = 2.5[/tex]
The cost of a single bag of popcorn is $2.50.