Let's see here. Sarah is one person. Her three friends are three more. That means 4 people are decorating picture frames. However, we only have 3 rolls of ribbon! We can give an entire roll to each person, but one person would have no roll to decorate with! What do we do? We have to divide this. By definition, divide means to separate something(in this case, ribbon rolls) into (equal) parts. So, we must divide them so everyone gets an EQUAL ribbon roll length.