Random Shit

Instead, respond as though whatever threat they are afraid of is fully valid, and offer whatever you can do to help- ask if they believe they are in danger of being hurt in any way, and work accordingly.

If they're overreacting, they'll usually realize and dial it back, self-correct and begin thinking a bit more rationally.

This is a given.

If the last digit is divisible by 2 (a.k.a. even), then so is the whole number.

If the sum of the digits is divisible by 3, then so is the whole number. The recursivity of this means that if the sum has multiple digits, you can add them up again until you get a single digit and see if it's 3, 6, or 9.

Like the rule for 2, but check if the last two digits are divisible by 4.

If it ends in 5 or 0.

If the rules for both 2 and 3 apply.

No shortcut. Alas.

Like the rules for 2 and 4, but check if the last three digits are divisible by 8. (Yes, this pattern keeps going for 16, 32, etc.)

Like the rule for 3, but the sum of the digits (or the sum of the sum of the digits, etc.) must be 9.

Another design is using 2 20x25x1 filters, taping them to the sides of the box fan and then to each other so they sort of make a triangle, then cutting cardboard to make a top and bottom to the triangle.

This was discovered as a more effective design during the 2020 US west coast fires.

https://tombuildsstuff.blogspot.com/2013/06/better-box-fan-air-purifier.html