The 20% markdown means a lower price than the usual. So, we have to find the 20% of the price and then calculate the difference between them. We know that 20% is 0.20:
Natural numbers are not closed under subtraction. In fact, any operation has no solution using natural numbers.
Natural numbers are closed under multiplication. In fact, any operation means to sum by itself times, and we already know that natural numbers are closed under sum.
Natural numbers are not closed under division. In fact, if doesn't divide , any operation has no solution using natural numbers.