Answer: $40 billion
Step-by-step explanation:
The change will be determined by the value of the Multiplier.
The Multiplier shows how much a change in government spending and exports will impart GDP.
Multiplier = 1 / ( 1 - MPC)
= 1 / ( 1 - 0.75)
= 4
Change in GDP = Multiplier * (Government spending + exports)
= 4 * (20 billion -10 billion)
= 4 * 10
= $40 billion