"Do the arithmetic. (250*0.18) / 4.7"
So "backing up" means copying the unused space also? If not the figures are different aren't they?
No. It doesn't mean copying the unused space. Did you actually look at, or try, the arithmetic calculation I posted? I guess not, or maybe you didn't understand it, so I'll explain it here.
You wrote
If i "back up" using DVD's about how many do i need for a 250 gig HD that is only is only 18% used?
The HD is 250 GB in size. 18% is the same as 0.18. So the amount of data to back up is (250 * 0.18) GB, or if you find it easier to understand, "Two hundred and fifty gigabytes times zero point one eight".
The answer is 45 GB.
You wanted to know how many DVDs that would fit on, so we divide that by 4.7 (The number of GB on a single layer DVD). The answer is 9.57. That means 9 whole disks and a bit more than half of a tenth disk. So you'd need 10 disks.
I thought the arithmetic was pretty simple and obvious, I'm sorry if I made a mistake thinking that.
If backing up meant copying the unused space, the number of disks woulb be 250 / 4.7 which comes out to around 54 disks. And that 0.18 would not be in the calculation I posted.