I can do this in a few seconds. Whenever I was in a long road trip or aboard a plane I used look at numbers and number sequences around me, and try to change them into other numbers by adding subtracting, multiplying or dividing. I usually work on other cities, and when I'm waiting for the plane (and my possible death, I'm scared of planes), I try to add the flight numbers and do math with them so I get the number 13.