maximum number of packs of 10 = 30 / 10 = 3 packs
Therefore, the different ways to but them are as follows:
3 packs of 10 and zero single pieces (30-3(10)=0)
2 packs of 10 and 10 single pieces (30-2(10)=10)
1 pack of 10 and 20 single pieces (30-1(10)=20)
zero packs of 10 and 30 single pieces (30-0(10)=30)
