Erica and Leah are two characters in "Bianca Hallo-whines." It is they and not Bianca who do the math in this segment.
They are two young friends of the sometimes-protagonist Bianca. Erica is the older sister, and Leah is the younger one. They live with their mother who spends the whole episode away for some reason. Bianca babysits them, in hopes that their mother will get home at five so the latter can take them trick-or-treating, and the former can go on a date with her boyfriend.
For Halloween, they dress as desserts: Erica as a cupcake, and Leah as an ice cream cone.
The girls show faces of concern when they hear Bianca groan over their mother's flight being delayed, meaning she has to take them trick-or-treating, and so she has to call off the date until the next night.
Bianca grouchily takes Erica and Leah trick-or-treating.
When they get home, Erica expresses a bad feeling about herself and Leah being the reason Bianca is not having a happy birthday. Leah comes up with the idea of sharing their candy with her. Erica and Leah count their pieces of candy: 12 each. They have no idea how 24 candy bars can be shared evenly between three people. Erica makes three notes for each group: one for Bianca, one for Leah, and one for herself. In every even-number pattern, Leah says at least the last two groups allowed. Twice, Erica and Leah add three pieces of candy to Bianca's group, three to Leah's, and three to Erica's. But then there are too few left to do it that way anymore, so they try giving one piece at a time. Ultimately, Erica counts the candies in her group: eight candy bars.
They call her to tell her they have a surprise, wish her a happy birthday, and apologize for ruining her birthday plans. Bianca is touched, thanks them, and uses the simile that they are "as sweet as this present." She apologizes to them for her crabbiness, but doesn't stop there. She asks to trade the licorice for chocolate, much to some sad smile on Erica's face, and a slight frown on Leah's.