Answer: $5 billion
Step-by-step explanation:
First find the spending multiplier which is a multiplier that shows how Aggregate demand increases as a result of additional spending.
Multiplier = 1 / (1 - Marginal propensity to consume)
= 1 / ( 1 - 0.8)
= 5
If the government wants to raise Aggregate demand by $25 billion, they should spend:
Increase in AD = Amount * Multiplier
25 billion = Amount * 5
Amount = 25 / 5
= $5 billion