discrete math

5.2  3b, 4, 5, 6, 7, 8, 12, 15, 16, 20

5.3  1, 4, 6, 8, 11, 12 (explain how both sides of the equality describe the number of strings of length n with 3 symbols), 14, 15

    Try to do these without using a calculator.
    In all cases where a number is asked for give a brief
    explanation saying how you arrived at your number.