Final answer:
The recessionary gap is four times the initial decrease in planned investment.
Step-by-step explanation:
A recessionary gap occurs when an economy is below its potential GDP, resulting in high unemployment. To calculate the size of the recessionary gap after a decrease in planned investment, we can use the multiplier effect. The multiplier is calculated using the formula 1/(1-MPC). In this case, the MPC is 0.75, so the multiplier is 1/(1-0.75) = 4. With a multiplier of 4, a decrease in planned investment will lead to a decrease in equilibrium GDP that is four times larger. Therefore, the recessionary gap would be four times the initial decrease in planned investment.
Let's assume there is a $100 decrease in planned investment. Using the multiplier, the recessionary gap would be $100 * 4 = $400. So the size of the recessionary gap after the fall in planned investment is $400.