What is the mimimum for food a day?

I found out my parents are sinking financially. So to help them budget, I did an audit of their financials. Long story short, my dad makes about $1800-2300 a month. He has about $1300 in guaranteed expenses, so we should assume he's going to have $600 or less available per month. So he's allowed to spend... show more I found out my parents are sinking financially. So to help them budget, I did an audit of their financials.
Long story short, my dad makes about $1800-2300 a month.

He has about $1300 in guaranteed expenses, so we should assume he's going to have $600 or less available per month.

So he's allowed to spend less than $20 a day on food and other discretionary.

What's the best way to make the most out of $20 a day for food?
Update: By the way, 20$ a day must includer him, his wife, and his 2 daughters
3 answers 3