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Fstoppers on YouTube: Printer Ink, It's a SCAM
(www.youtube.com)
This is a most excellent place for technology news and articles.
How? He can't weigh it without also weighing the foam.
Could possibly weigh the foam thats soaked in ink then wash the foam to remove the ink and dry it out then weight it again and compare. Might not be very accurate but it would be an attempt at closer numbers at least.
Even then you're supposedly paying for usable ink. I don't think the ink will all drop down from the foam.
This can be tested still.