Tax question.

[progree]

3. Assuming you're single and take the standard deduction

And assuming that all of your Social Security is taxed (actually the maximum is that 85% of it is taxed):

73,000 Income not including casino
-12,200 standard deduction
------------------------------
60,800 taxable income not including casino.

It is the Taxable Income, not the gross income, that one enters the tax tables with.

A taxable income of $39,475 to $84,200 is the 22% tax bracket (2019 taxes)

They chose terminology that was designed to be confusing. But the idea is that the first $12,200 of your income is meant to be "free", and the remainder above that is "taxed", hence, "taxable income".

With the $18,000 casino winning, your taxable income is 60,800 + 18,000 = $78,800

Still within the 22% tax bracket. So the casino winning is all taxed at 22%.
$18,000 * 22% = $3,960.

Plus state taxes ....

That 15% or more of your Social Security is NOT taxed won't change this calculation -- the casino income will still all fall within the 22% bracket. (Well, I'm assuming you're around the 85% SS taxation point when I say that)

EDIT - Ooops, if you are over 65, your standard deduction is an additional $1650, so it is $13,850 instead of the $12,200 I showed in the above calculation. But even with that adjustment in your favor, your casino winnings will still all fall within the 22% bracket.