a shop stocks six types of phone battery (b1, b2, b3, b4, b5, b6) and a total inventory of 60 phone batteries. assume that the store can get at least 60 of each type of battery. in how many ways can the inventory of 60 batteries be distributed over the six di? erent types if:
(a) there are no restrictions on the number of battery of any type?
(b) the shop must stock at least ? ve batteries of each type?
(c) the shop stocks either b1-type batteries or b2-type batteries, but not both (with no restriction on the type b3, b4, b5, b6 batteries)?