Allow an interval endpoint on either side of the fixed bound

X is normally distributed with mean 10 and standard deviation 2.

Find $P(9<X<13)$ and $P(X<12)$.

Find the possible values of a if the probability that X lies between a and 12 is 0.15.

If three values are taken at random, find the probability that two are less than 12 and the other greater than 12.