Suppose that currency in circulation is $$600$ billion, the amount of chequable deposits is $$900$ billion, and excess reserves are $$15$ billion. Next month, the central bank conducts an unusually large open market purchase of bonds held by banks of $$1400$ billion due to a sharp contraction in the economy.

Required: Calculate $-$

a$.$ the excess reserves,

b$.$ the excess reserve ratio,

c$.$ the money supply, and

d$.$ the money multiplier?