Mental math can be used to estimate a 15% tip on a taxi ride that costs $23.25 by first calculating 10% of the cost. To do this, move the decimal point one place to the left and multiply the result by 10. In this case, 10% of $23.25 is $2.33. To calculate 15%, add half of the 10% amount to the 10% amount. In this case, half of $2.33 is $1.165, so the 15% tip would be $2.33 + $1.165 = $3.495. Rounding up, the 15% tip would be $3.50.
