\[\text{Average money holdings }=\frac{Y}{2 N}\]
\[N* = \sqrt {\frac{{iY}}{{2F}}} \;\]
\[(M / P)^{d} =L(i, Y, F)=\sqrt{\frac{Y F}{2 i}}\]
where Y is total spending over a year, i is the interest rate on savings account, N is the
the number of trips consumer makes to the bank to withdraw money from the savings
account, and F is the cost of a trip to the bank.