Question: hexadecimal digits are 0 1 2 3 4 5 6...

Hexadecimal digits are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F. (Please explain briefly)

1. How many hexadecimal strings of length ten have at least three E’s?
2. How many hexadecimal strings of length ten have exactly two A’s and at most two B’s?
3. How many hexadecimal strings of length ten have six digits from the set 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and four digits from the set A, B, C, D, E, F? No repetition is allowed.
4. How many hexadecimal strings of length ten have exactly four A’s and all remaining digits distinct from each other?