限界的な消費傾向

「雇用・利子および貨幣の一般理論」に書いてある説明をわかりません。
Keynes claims that, given employment of 5,200,000, an additional 100,000 people added via a public works project would lead to a new employment level of 6,400,000. This gain of 1,200,000 eludes me. The multiplier at the 5,200,000 level of employment is 50, and the amount of income invested is 3120. If the 100000 is to be taken as an investment amount, then the rise should be 5,000,000. I am not sure what is to be considered to be delta(I) in this example. If we divide 100000 by the multiplier, obtaining 2000. Then, if we add the gain in investment from the difference between the level 5300000 and 5200000, (6000-3120) and multiply this by 50, we obtain 244000. So the increase in employment is much less than 1200000.
However, if we use 100000 as delta(I), the multiplier must be reduced substantially for the numbers to be coherent.