How to calculate the spending multiplier from a given set of equations? So i have this question:

I go along and get 


Then i need to calculate the effect on the optimal output is G increases by 80:

And on the answer sheet it states that the spending multiplier is:



From my knowledge i know that 


Now how come that the spending multiplier is 1/0.4? Where are they getting the 0.4, which should be 1-c1-d1 from the original equations?

A friend told me that the 1/0.4 is derived from 1/1-c1(1-t) 

then 1/1-0.8(0.75)= 1/0.4. He said that since there is no d1 in the equations ( in the investment equation) we don't use it.

Can somebody explain the meaning behind this 1/0.4) Thanks!
Your friend is right. If you look at the initial investment equation, and comparing it to your definition, you have:
$$I=900+(0)Y-50i$$
So $d_{1}=0$ and $c_{1}=0.8(0.75)$ and thats why you get the spending multiplier as $\frac{1}{0.4}$