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One year after being bought for $44 billion, X is worth $19 billion
(arstechnica.com)
This is a most excellent place for technology news and articles.
Companies such as Fidelity give an estimate on how much they are value after analyzing the finances of the company I think.
Fidelity does this because they invested in Twitter alongside Musk, and as a bank they are required by law to disclose the current value of their assets. They come up with a value of their stake, then the rest of us divide that by the percentage Fidelity owns to get the value of the company.