How to calculate 20 %?

Sorry....

Very simple question...

to increase a number by 20%, you multiply by 1.2    eg   100 x 1.2 = 120
But to reduce by 20%  100 / 1.2 =  83.33333

What operation should I do to obtain 80 (100 - 20%)?

Thanks

100 x (1 - 20%)
=100 x 0.8

Think of a number from 1 to 10, divide by that number, add the age of your dog (in days), multiply by the square root of (the number you first thought of divided by itself), subtract (today's date minus your dog's date of birth(in days)) and divide by ((the square root of 25 squared) over 5)

Then take the number you're looking for 20% of and multiply it by the above paragraph.

How do you know he has a dog PP ?

Fab,
  if you're looking for the way to find the origional price
and all you've got is 'now 20% off, new price  £80.00'
just divide by the reciprocal  ie   80/.8  = 100.
If it was 25% off it's 80/.75 =106.67

Well if he doesn't have a dog then he has no business of being a gambler also :p and what I really mean is that anyone who asks this kind of question should run as quickly as he can as far as he can from betting industry and hope that the place where he arrives is third grade math lesson.

stop tapping the tank

20% x 0 = 0

FINE AS FROG HAIR
11 Feb 13 04:53

How do you know he has a dog PP ?

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I'm stalking him. I'm planning to dissect his brain to build a computer.

"I'm stalking him. I'm planning to dissect his brain to build a computer. "

Ok, that's obviously answering his original question "What operation should I do to obtain a ZX 80"