Nvm.
If we indicate with $\displaystyle M$ the amount of money in circulation and with $\displaystyle \delta$ the tha quantity in per cent that circulates throgh the banks in a day, the quantity of new currency generated in a day is...
$\displaystyle x^{'} = \delta (M - x)$
... that can be written in standard form as...
$\displaystyle x^{'} + \delta\cdot x = \delta \cdot M$ (1)
The (1) is a linear first order DE that can be solved in usual fashion...
Kind regards
$\displaystyle \chi$ $\displaystyle \sigma$