The sample problem is:

Karen is Married filing Joint Return with 2 dependents under 17. She did not check box 2 on her W-4 form. She is paid biweekly and forthis pay period her Taxable Wages for Federal W/H is $2,500. There are no pre-tax deductions. Using either the IRS 2020 Pub 15-T or the IRS Income Tax Withholding Assistant Excel spreadsheet indicate the Fedeeral W/H amount.

The sample has $18.15 as the Federal W/H amount? I cannot figure out where they got this amount from? Please explain the process so I can determine what I am missing? Huge Thanks.