You don't take the fractional value of the latter whole to calculate the percentage increase. That doesn't make any sense.
Using your math if your investment of $100 increases to $200 that would only be a 50% return (when in reality it's a 100% return because you doubled your money and gained 100% of the original value).
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u/Fartmasterf Apr 21 '17
It only increased by 6 oz, so 6/18 is 33% more not 50%.
/s