A1. Shawn's password to unlock his phone is four digits long, made up of two 5 s and two 3s. How many different possibilities are there for Shawn's password?
Solution: This is the number of ways to choose 2 spots out of 4 , thus there are $\binom{4}{2}=6$ possibilities. Alternatively one can list all the possibilities: $3355,3535,3553,5335,5353,5533$.
Answer: 6 .