Let N be a positive multiple of 5 . One red ball and N green balls are arranged in a line in random order. Let P(N) be the probability that at least \dfrac{3}{5} of the green balls are on the same side of the red ball. Observe that P(5)=1 and that P(N) approaches \dfrac{4}{5} as N grows large. What is the sum of the digits of the least value of N such that P(N)<\dfrac{321}{400} ?
Answer Choices
A. 12
B. 14
C. 16
D. 18
E. 20