Usually such magnets are made out of iron. If that is the case, I would expect its density to be similar to that of iron (approximately 7900 kg/m3).
20% of 7900= 20% * 7900= 0.2 * 7900= 1580
To calculate 7900 percent of 8000, calculate (7900 / 100) x 8000.
7900 lots of them.
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There are 1000 grams in one kilogram. Therefore, 7900 grams is equal to 7900/1000 = 7.9 kilograms.
Wikianswers calculator says 7900 * 3.14 = 24806.