Question

# How many five-letter sequences are possible that use the letters q, u, a, k, e, s at most once each?

Probability and combinatorics
How many five-letter sequences are possible that use the letters q, u, a, k, e, s at most once each?

Answer: $$\displaystyle{6}\cdot{5}\cdot{4}\cdot{3}\cdot{2}={720}$$