Based on the information given:  

  ⇒ there are 12 people --> Emily, Delvenal, 10 other friends

      ⇒ the pan of brownies needs to be evenly split among them.

Consider the possibilities:

• One option: cut the pan of brownies into the 12 pieces

             ⇒ so that everyone gets one piece of brownie

• Second Option: cut the pan of brownies into 24 pieces

             ⇒ so that everyone can gets two piece of brownie

In other words, you can cut a number of pieces that must be divisible by 12 in other to split equally within the group.

Hope that helps!