Final answer:
To narrow the trade deficit from -200 to -100 billion dollars, national savings must increase by 100 billion dollars, provided that investment and other factors remain constant.
Step-by-step explanation:
To reduce a country's trade deficit from -200 to -100 billion dollars, one needs to understand the connection between the trade balance and national savings. The trade balance, defined as exports (X) minus imports (M), can also be represented as Savings (S) minus Investment (I). Thus, to reduce the trade deficit (X-M), either savings must increase, investment must decrease, or a combination of both. In this case, if the goal is to reduce the trade deficit by 100 billion dollars solely by increasing savings, then national savings must rise by that same amount, 100 billion dollars, assuming there are no changes in investment or other factors.