How much is 13 percent tax on 39.99?

Oh, dude, math time! So, to find 13% of 39.99, you just multiply 39.99 by 0.13 (which is the decimal form of 13%). That gives you a whopping $5.19 in tax. So, if you're buying something for $39.99, add that $5.19 to the bill, and you're good to go!