In a population of 10,000 individuals, 400 express a recessive trait. how many heterozygotes are there expected to be?

We will use the Hardy-Weinberg equation ; p2 + 2pq + q2 = 1
q2 represent the frequency of the recessive trait in the population which is 400/10000 = 0.04. Then find frequency of the recessive allele.
q = 0.2. While we understand that p+q =1 ; p = 1-q ; p = 1-0.2 =0.8.
2pq represents the frequency of heterozygous trait in the population. Therefore 2*0.8*0.2 =0.32
0.32*10000 = 3200

The answer is ozone layer.