PHP: basic percentage calculation [closed]

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I have $sum and need to calculate the sum minus 15% and then I need to display the original value again.

$sum = 199;
echo "value 1: $sum";
开发者_Python百科$sum = $sum*0.85;
echo "value 2: $sum";
$foo = $sum*1.15; 
echo "value 3: $foo";

This is the result I get:

value 1: 199 
value 2: 169.15 
value 3: 194.5225

Please tell a very stupid person why this formula is obviously wrong and doesn't return 199 in the last output.

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I agree with everybody about just keeping the original variable. However, the single-variable answer is this:

1.99 * .85 = 169.15
169.15 / .85 = 1.99

Change operator ;)

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It's because 1/1.15 is not equal to 0.85.

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Basically, you starting off with 199 and finding 85% of that (the subtract 15%) which is correct. But now, when you're trying to gain the 15% back you're doing so based on the value of 15% of 169.15 (25.3725) not 15% of 199 (29.85).

As mentioned in another answer, if you need to original number back, keep it in a variable and use a temp variable to hold the percentage.

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$sum = 199;
echo "value 1: $sum<br />";
$sum2 = $sum*0.85;
echo "value 2: $sum2<br />";
echo "value 3: $sum<br />";

So, who's the genius?

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Percentages don't work that way. You can't just do the reverse to the result of the first operation. You should just store the original number and use that.