Some are easy. 1,2,5,10 should never be an issue. Neither should 3 honestly so we're halfway there. 4 is just doubling 2. 8 is just doubling 4. 70%. 9 has a trick. What ever you multiply it by, subtract one, and then make the 2nd digit whatever makes the 1st digit sum with it to 9. For example 9x8=7+(2)=72. Now you're 80% there. All you gotta do it not be lazy and memorize your stupid 6's and 7's. 