Q2.Jun 2014 iTTrain explanation

Could you please explain how the calculation of Contribution for each delegate group is calculated.
I am confused on why with 4 delegates the contribution is 28,000 and 4 delegates is 56,000. Is the MR per one doubles for every additional delegate?

All financial figures are in $

Number of delegates Contribution Fixed overheads Operating profit (loss)

3 0 65,000 (65,000)
4 28,000 65,000 (37,000)
5 56,000 65,000 (9,000)
6 84,000 65,000 19,000
7 112,000 65,000 47,000
8 140,000 65,000 75,000
9 168,000 65,000 103,000

Thanks in advance.