Question: How many strings of four decimal digits (Note there are 10 possible digits and a string can be of the form 0014 etc., i.e., can

How many strings of four decimal digits (Note there are 10 possible digits and a string can be of the form 0014 etc., i.e., can start with zeros.)

a) have exactly three digits which are 9s?

b) do not contain the same digit twice?

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