What is the least common blood type in this population assuming HW equilibrium?

AB???

Type AB is the least common blood type for this population--8% of the population will have type AB blood.

H-W equation for 3 alleles is:

1 =  p^2 + r^2 + q^2 + 2pq + 2pr + 2qr

Let's say that "p" is allele A, "r" is allele B, and "q" is allele O.

The frequency of each individual blood type is given by:

AA:  p^2 (frequency of A squared) = 0.2^2 = 0.04
BB:  r^2 = 0.2^2 = 0.04
OO: q^2 = 0.6^2 = 0.36
AB: 2pr (frequency of A times frequency of B) = 0.08
AO: 2pq = 0.24
BO: 2rq = 0.24

This allows you to calculate the frequency of each genotype.  Because some genotypes are the same phenotype, you have to add the frequency of each different genotype that produces that same phenotype:

Type A:  AA + AO = 0.04 + 0.24 = 0.28
Type B:  BB + BO = 0.04 + 0.24 = 0.28
Type O:  OO = 0.36
Type AB:  AB = 0.08

Type AB is the least common blood type in this population.