let's say the least amount of money is "x", therefore whatever "x" is, it has to be "greater than or equals to 500", thus x ⩾ 500.
so, let's add "x" to the current balance including debts.
[tex]\bf x\ge 500
\\\\\\
\stackrel{minimum}{x}+\stackrel{current}{727.29}-\stackrel{\textit{cheque for payment}}{248.50}\ge 500\implies x+478.79 \ge 500
\\\\\\
x\ge 500-478.79\implies x\ge 21.21[/tex]