Interestingly enough in the example he gave 303 x 13 I actually didn't look at the last digit to see that 5074 is wrong, I did an instant mental calculation of

Interestingly enough in the example he gave 303 x 13 I actually didn't look at the last digit to see that 5074 is wrong, I did an instant mental calculation of 300x13=3900 and saw that 5074 can't possibly be right before reading on. So everyone has a different spidey sense.