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Let’s check out your problem:

(a) To log on to a certain computer account, the user must type in a $3$-letter password. In such a password, no letter may be repeated, and only the lower case of a letter may be used. How many such $3$ -letter passwords are possible? (There are $26$ letters in the alphabet.)$\newline$$\square$$\newline$(b) From a collection of $52$ store customers, $2$ are to be chosen to receive a special gift. How many groups of $2$ customers are possible?$\newline$$\square$