## A Conversation for Divisibility

### Divide by

aka Bel - A87832164 Started conversation Jun 12, 2007

Hi Icy, can you believe that I thought of your entry yesterday when I wanted to divide 470 by 24 to compare it to 570 divided by 30.

Guess what: I couldn't remember any of the rules because I didn't learn them.

Fortunately, it's not too difficult to divide the numbers I had, but it could have been worse, and then your entry would come in handy - if I managed to remember it all.

Great entry, very useful for everyday maths!

### Divide by

Icy North Posted Jun 12, 2007

Hi Bel

Well the entry won't really help you compare ratios like that - it can only tell you if 470 is exactly divisible by 24 (which it isn't as the last two digits aren't divisible by 4)

Numbers like 470 are easy to factorise - it's 47 * 5 * 2, so it doesn't have all the factors you would need to divide it by 24 (2, 2, 2 and 3)

570 is divisible by 30, though. It ends with a zero, and it's digit sum is 12, which is divisible by 3. 570/30 = 19

I'm not sure what the best techniques are for comparing ratios, but if I find out, I'll post it on h2g2 first!

Icy

### Divide by

aka Bel - A87832164 Posted Jun 12, 2007

Well, I managed that myself because the digits weren't too long, so knowing that 47 doesn't divide by 24 wasn't a problem, and it was easy to say that it was slightly more than the 19 of the second example, but less than 20. But if ever I come across more complicated numbers, it would probably help to know whether or not they are exactly divisible by another one.

### Divide by

Icy North Posted Jun 12, 2007

I think I understand. Knowing that a number is divisible by another tells you quickly whether it's worth doing the division. (I think)

### Divide by

aka Bel - A87832164 Posted Jun 12, 2007

Yes, that's about what I meant. It's sort of pondering: do I do the maths in my head or do I search for the calculator?

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