Nine identical sponges cost 7.20 dollars. At what price would a 12-sponge pack have exactly the same value per sponge?

Explication

The nine-pack costs 0.80 dollars per sponge, so 12 sponges at the same rate cost 9.60 dollars. Adding the three extra sponges as though they each cost one dollar would give 10.20 dollars and change the unit price.