Bio- index of diversity?!

Attempting question 7 of the Biol2 June 12 paper and no idea..

It says 'use the formula
d=N(N-1)
Σ (n-1)
And I don't see any difference between N and n (N= total number of organisms of all species, n=total number of organisms of each species)

Any ideas?

I'm not really sure what you're trying to say for the formula, but do you perhaps mean $d=\frac{\sum n(n-1)}{N(N-1)}$?

n is the number of organisms in each individual species and N is the total number of organisms (regardless of species). Let me give you an example to make this clearer:

Say you have three species present and you had 10 of species A, 20 of species B and 15 of species C. You would calculate n(n-1) for each species, ie:

for A: 10x(10-1) = 90
for B: 20x(20-1) = 380
for C: 15x(15-1) = 210

This means Σn(n-1) = 690

N is the number of all organism ie 10 + 20 +15 = 45. So N(N-1) = 45x44 = 1980.

In this case your diversity index would be 690/1980 = 0.35