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August 2, 2015

Homework Help: URGENT MATHS

Posted by Sindy on Friday, February 1, 2008 at 1:36am.

I got 2 questions and I've been stuck on them for days and its due in 2 days.

1. For a quick way of doing multiplication, you can do:

for example: 81 x 89=? First you do 1x9=9 in which case you have to add '0' in front of it to make it 09. Then you add 1 to any of the 8s and multiply the tens digits making 72. You add the digits 72 to 09 making 7209.

Another one: 64x66=? 6x4= 24 and 6+1=7. Then I go 6x7=42. So the digits 42 and 24 makes 4224.

The question is WHEN can I do this. I already know that the last 2 digits must add up to 10, but what number does this method end? Over 100?

Second question. Why does 9+4=1? Because of clock arithmetic. 9o'clock + 4 hours= 1 o'clock, and the modulus is 12 for 12 hours for one cycle. I need to find out what other things uses a particular modulus. I've managed to think of a month(31,30,29,28) days.
What else?

Thanks heaps!

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