The numbers of each group are 1, 2, 3, 4, 5 and 6. .......

Each group of molecules starts with 1 and the denominator is a number. The numerator increases and the denominator decreases.

F(n)=2/2007, which means the second in the 2007 group.

So n =1+2+...+2006+2 = 2006+2006 * (2006-1)/2+2 =1003 * 2007+2 = 201.