Are about the money supply, M, which is the total value of all the cash and checking account balances in an economy. It is determined by the value of all the cash, B, the ratio, c, of cash to checking deposits, and the fraction, r, of checking account deposits that banks hold as cash:

(a) Find the partial derivative.
(b) Give its sign.
(c) Explain the significance of the sign in practical terms.

∂M∕∂r

M = c+B. c+r