Greta wanted brownies for her birthday.  She made a batch for herself; one dozen cream cheese swirl brownies.  At her office, they threw her a party and sent her home with 1/2 a dozen brownies.  When she arrived home, her friends were there to throw her a surprise party and had 4 dozen brownies waiting.  During the party, 1 1/2 dozen brownies were eaten.  How many individual brownies did Greta have left over from the entire day?
Give me a detailed solution.