How do you add 18% gratuity after tax is added?

Actually, you NEVER add gratuity to the tax, only the basic bill. Multiply $840 * .18 to get the gratuity -- $151.20 in this case.

your tax on the expense was 6%. So just multiply the tax amount by 3. simple rule 3 to 4 times the tax amount = tip, this covers most of the states (depending on total tax rate).

Generally you would just double the tax. That usually comes to about 16%. If you want to be exact, multiply the total by .18. Judy is wrong, you would multiply by 1.18 to get the total plus gratuity.

890.40 * .18 would be the gratuity, or 890.40 * 1.08 would be the total including gratuity