Carol and David shared a pizza with 10 equally-sized slices. Carol wanted a plain pizza, but David wanted mushrooms on two-fifths of the pizza. The cost of a plain pizza was $10, and there was an additional cost of $4 for putting mushrooms on two-fifths. David ate all of the slices of mushroom pizza and three plain slices. Carol ate the remainder. Each then paid for what he or she had eaten. How many more dollars did David pay than Carol?
6
7
8
9
10
