Need help with following probability questions:

1.Alice wants to create 5-letter security codes using 10 different letters so that a code cannot contain a letter that appears more than 3 times. How many choices are there? (Note that order matters.)

2.Alice flips a coin 10 times. As a result, she gets a sequence of letters H (if head appears) and T (if tail appears). For example, HHTTHTHHHT is such a sequence.

a. How many different sequences are possible as a result?

b. How many different sequences contain exactly 4 H letters?

c. How many different sequences contain at least 2 H letters?