Can someone tell me how to do this and how to find the answer?

in one kind of butterfly, brown color is caused by a dominant allele, b, and a white color is caused by a recessive allele, b,. the table shows the observed numbers of phenotypes in a population of 100 butterflies.

of individuals
brown 84
white 16

what is the frequency of the heterozygous genotype in this population of butterflies?
a: 0.24
b: 0.36
c: 0.48
d: 0.60