'

9) How many strings of length 10 over the alphabet {a, b, c, d} have exactly 3 a\'s?

a. (103)

b. 47

c. (103)⋅37

d. (103)⋅47

a.

b.

c.

d.

10) How many strings of length 12 over the alphabet {a, b, c, d} start with "aa" or end with "aa" or both?

a.  

b.

c.

d.