Nikki used the calculations shown to determine whether a carton of 12 eggs or a carton of 18 eggs was the better buy.

A carton of 12 eggs cost $2.39, and a carton of 18 eggs cost $3.39.
Unit price of the 12-egg carton = ($2.39)(12 eggs) = $28.68 per egg
Unit price of the 18-egg carton = ($3.39)(18 eggs) = $61.02 per egg
The 12-egg carton has the lower unit cost, so it is the better buy.

What was her first error?
She should have divided the