1 girl bring -3 difference to the mean and 1 guy adds +9 to the mean of girls
so 2 girls would constitute = -6 and 1 guy would add +9 resulting in +3 which is exactly the difference between girls height and mean
so for 2 girls there has to be 1 guy and thus answer choice A

Let's say boys be B and girls be as G.

Avg. Weight formula
65 = [71*B + 62*G]/[B + G]

65(B + G) = 71*B + 62*G

3G = 6B
G = 2B

Doesn't this mean 1 girl is equal to 2 boys?

Yes. For a total of three persons.

One female and two males

This is not correct.
B = number of boys
G = number of girls

G = 2B tells us that the number of girls is equal to TWICE the number of boys.
We can also verify this by finding values of B and G that satisfy the equation G = 2B
For example, G = 10 and B = 5 satisfies the equation G = 2B (notice that the number of girls is twice the number of boys)

Another way to avoid misreading G = 2B is to rewrite it in ratio form.
Take: G = 2B
Divide both sides by B to get: G/B = 2
This is the same as: G/B = 2/1
Or we can write: G:B = 2:1
This tells us that there are two girls for every one boy (i.e., the number of girls is twice the number of boys)