Inflation and Quantity theory of Money. Adapted from Jones Q3 Ch 8: What is the key endogenous variable in the quantity theory? (Read chapter 8 if this isn't clear) Explain the effect on this key variable of the following changes (holding everything else equal): (a) The money supply is doubled (b) The velocity of money increases by 10% (c) Real GDP rises by 2% (d) The money supply increases by 3% while real GDP rises by 3%