A researcher determined that the ratio of males to females in a healthy herd of deer should be 3 to 7. If a population has 70 deer, how many female deer should be in the herd in order for the herd to be considered healthy?

The correct answer is: 49

Given 
Population = 70 = 3 + f = 210 = 3 (m +f)
3m + MF = 210,
Substitute equation 1 in 2
Population = 70 = 3 + f = 210 = 3 (m +f)
3m + MF = 210,
Substitute equation 1 in 2
3m + (7m) = 210, 10m = 210, m = 21.
Substitute m in equation 2
(21) + f = 70, f= 49.
So, option D.