Answer:
$6.00
Explanation:
A fairly easy way to do the mental calculation of a 15% tip is to realize that ...
... 15% = 10% + (1/2)·10%
Of course, you can compute 10% by moving the decimal point 1 place left, so ...
... 10% of $40.00 = $4.00
Half that is $2.00, so the 15% tip is $4.00 +2.00 = $6.00.