How many four-digit numbers can you get out of
balls in the box:
of red ones and
of blue ones. In how many ways can two balls of the same color be picked?
In how many ways can you pick a captain and his deputy in a group of
cats. In how many ways he can take
of them in his hands (one - in left hand, another - in right)?
Three passengers walk into the elevator of the -floor building (ground floor is not included). In how many ways all three passengers can get out on different floors?
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