Delicious donuts sells their donuts in different sizes

6 for $2.10

12 for $5.49

18 for $7.20

24 for $12.00

Which box is the better buy?

6 for $2.10

Because you can get two boxes (total of 12 donuts) for $4.20 instead of $5.49

Or three boxes (18 donuts) for $6.30 instead of $7.20

Or 4 boxes (24 donuts) for $8.40 instead of $12