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# 8 Useful Math Hacks That They Didn’t Teach You In School

### 14 Responses to "8 Useful Math Hacks That They Didn’t Teach You In School"

1. Johan says:

How do you get to the numbers on the date thing?

2. Daniel says:

don’t see any working for that date day of week one. so not genius – genius is communication too!

3. tim says:

you need to explain the what day of the week it will be or was a lot better cause I don’t see any explanation at all.

• Catherine K says:

tim I found a website with the instructions and it was not straight forward. First you memorise a table of numbers for the months which need January and February adjusted if it is a leap year. The days was straight forward, the 20th day of the month will be the same day of the week as the 6th day. For the year you need to know if it is a leap year and depending on which century either add or subtract something by then I had lost interest because it seemed like a lot of facts you had to memorise to be able to do something which is in my opinion essentially a party trick.

4. Karla says:

The answer is “ask Siri”! I just looked it up, the calculation is very complicated, so didn’t show it. Not really a Hack!

5. Joe Schmoe says:

#2 doesn’t work, unless I’m missing something – I’m getting 9×9 is 82, obviously not correct

• Donna says:

Hi, Joe, you forgot to multiply the top results not add. Eight digits below = 8 and two digits above multiplied 1×1= 1 to get 81. Your lapse helped me solidify the hack so thanks for making the mistake. I learned too.

• MC says:

The fingers on the “top” have to be multiplied not added. So you get for 9×9 the 8 fingers from the touching ones down and above you have 1×1 -> 81.
😉

• Rou says:

#2 is working. 1×1=1 (missing something!)

• CM says:

You have to multiple the number of fingers on the top, not add them. So for 9×9, you have to fingers above the touching fingers, using the formula that is 1×1 = 1. Then you count the number of fingers touching and below for the tens place which is 8, giving you 81.

• CM says:

oops, I obviously meant “two fingers” not “to fingers”

• tom says:

9 x 9… You got that you add the eight fingers for the 80, then you added the 1 finger plus the 1 finger. You should have multiplied them 1 x 1 = 1. that would give you 81

• bella says:

joe i am 11 and even i know that 9×9=81 duuuuuuhhhhh

6. Well you should says:

You should by 11. Duhh