What will be the additional investment? I recently encountered upon this question:

Suppose, in  an economy,autonomous consumption is Rs 100 crore and autonomous investment is Rs 60 crore. Imagine that in this economy,with every increase in income 80 °/° of it is spent on consumption .  If the full employment level of income is Rs 1000 crore,what will be the additional investment?  

So, the MPC is 0.8 and so the multiplier would be 5 times. 
The consumption function is C =  Autonomous consumption + MPC.y
Hence, C = 100 + (0.8)1000 = 900 crore.
But then given that the multiplier is the ratio  change in income to change in investment, how to proceed further?
I think you use the inverse (1-MPC) of the MPC to derive the marginal propensity to save (MPS) and create the investment function.  So, I= 60 + .2*1000 = 260, or an additional 200 from the autonomous level.