If the MPC is .75 then the MPS is .25. as MPC + MPS must equal 1.
The formula to find the spending multiplier is 1/MPS or 1/1-MPC.
1/MPS = 1/.25 = a spending multiplier of 4
If investment increases by 20n, then the increase in GDP will be multiplied by 4.
20bn x 4 = 80n increase in GDP.