Tax Redistribution Problem

In a certain economy, let the total amount of circulating money be denoted by $M$.

The economy has a unique tax policy where the government imposes a tax of $p$ percentage on the total money $M$ at the beginning of each year.

After applying the tax, the remaining money is redistributed back to the citizens in a way that each citizen receives a uniform amount $r$.

1. The total number of citizens in the economy is denoted by $N$.

2. At the end of the year, if $M$ is reduced to 80% due to the taxing effect, what is the formula to determine $r$ in terms of $M$, $N$, and $p$?