Facebook, Inc. had no debt on its balance sheet in 2014, but paid $2 billion in taxes. Suppose Facebook were to issue sufficient debt to reduce its taxes by $290 million per year permanently. Assume Facebook's marginal corporate tax rate is 39% and its borrowing cost is 4.1%.

a. If Facebook's investors do not pay personal taxes (because they hold their Facebook stock in tax-free retirement accounts), how much value would be created (what is the value of the tax shield)?

b. How does your answer change if instead you assume that Facebook's investors pay a 20% tax rate on income from equity and a 39.6% tax rate on interest income?

ANSWERS ARE IN BILLIONs