Answer:
At the end of these four years, the federal government's public debt would have increased by $20 billion
Explanation:
Year 1:
Budget deficit=$40 billion
Year 2:
Budget deficit=$50 billion
Total budget deficit=$90 billion
Year 3:
Budget surplus=$20 billion
Year 4:
Budget surplus=$50 billion
Total budget surplus=$70 billion
Budget surplus - Budget deficit= $90 billion - $70 billion
=$20 billion
At the end of these four years, the federal government's public debt would have increased by $20 billion