Constance

Specifically, for the number 7. Knowing shortcuts for numbers higher between 10 and 20 would be neat too. I already know the shortcuts for 2-6 and 8-10. I've wondered this ever since I was in school. Thanks!

Let's say in this hypothetical situation that it's possible scientifically, and there was a complete 0% chance of having any ill effects, death included. If you were FORCED to, within 3 days, pick a member of the opposite gender alive today (opposite to you by your birth gender) to have a brain transplant with (and so take over his/her body as your own), and if you didn't choose w/in 3 days someone would be chosen for you at random, who would you choose? Or would you choose to let it be random?

Going through possible factors isn't practical, and can take forever, so that's not a viable choice. Also, any prime number has to end in 1, 3, 7, or 9, because otherwise it's not prime. (Even numbers are divisible by 2 and 5 just means it's divisible by 5)