Math Rules (yes - yes it does) may be a bit misleading. These are tips to help make some operations a bit easier. The National Council of Teachers of Mathematics had an article in November of 2015 that talked about the 12 rules that expire in the middle grades. Kind of significant for mathematics - a place where the rules are not supposed to change. Of course these were more teaching techniques than rules. I have always been of the opinion that most of these shortcuts do more harm than good. Why should we teach stuff that we will later tell you to forget? Anyway - these rules are forever (I think).
If the sum of all the digits of a number is divisible by 3 then the total is divisible by 3. So is 1,459,473 divisible by 3? Well 1+4+5+9+4+7+3 = 33 and 33 is divisible by 3 - so, yes 1,459,473 is divisible by 3. Incidentally, if the sum is divisible by 9, the number is divisible by 9. Is this a great math rule or what?
My father was an accountant - so this was one of his favorite math rules.
Let's say you're adding up a column of figures and you get a different answer the second time through. If the difference between the two sums is divisible by 9, you probably made a transposition error. Meaning - for instance - instead of 41 you typed 14. The difference here is 27 which is divisible by 9. Transposition errors are real easy to do if you are typing in long lists of numbers into a spreadsheet or using a ten key.
When you are multiplying, think of it as stacks. So, 9 times 23 is 9 stacks of 23 things. Now, to multiply, you know that 10 times 23 is 230, subtract one stack of 23 and you get 207.
To divide by 5, multiply by 2, then move the decimal one space to the left. So, 13 divided by 5 is 2.6.
In other words, double the number and divide by 10.
Say you're multiplying 17 times 8. Double the 17 and half the 8. You get 34 times 4.. double the 34 and half the 4. You get 68 times 2 or 136.
Of course, this assumes doubling numbers is something you're good atg.
All I can say here is that calculators are extremely cheap - heck, your phone probably has one built in. I don't know of a trick for multiplying by 7. Of course, if it's an even number times 7, the trick in Math Tip #5 might work for you.
Most of the waiters I know work pretty hard for little pay (I used to wait tables, can you tell?) So my first suggestion is to double the amount and move the decimal one place to the left. So, if a meal costs $19, double it and get 38, move the decimal and your time is 3.8 or $3.80. For a 20% tip.
O'k, your expense account only allows 15% tips. Move the decimal to the left on place and add half. The $19 meal again, we'd move the decimal and get 1.9, half of that is .95 add them together and you get $2.85.