combinations with repetition

A shop has 4 distinct flavors of ice-cream. One can purchase any number of scoops of any flavor. The order in which the scoops are purchased is inconsequential. If one wants to pur...

The number of $$4$$ digit numbers having their digits in non-decreasing order (from left to right) constructed by using the digits belonging to the set $${1, 2, 3}$$ is____________...

In how many ways can $$b$$ blue balls and $$r$$ red balls be distributed in $$n$$ distinct boxes?