Answer:
increase the public debt from $460 billion to $480 billion
Explanation:
Other things equal an increase of treasury bonds from $100 billion to $120 billion in the economy would:
"increase the public debt from $460 billion to $480 billion"
Since the public debt consists of the debt instruments issued by the US goverment, thus, Treasury Bills, Tresaury Notes, Treasury Bonds and U.S. Savings Bonds would constitue public debt, the sum of which would be $460 billion and an increase in treasury bonds from $100 billion to $120 billion would increase the public debt by $20 biilion to $480 billion.