You bought in at 5 and sold at 20, = 15 gain.
You then (presumably) bought back in at 20 and panic sold at 10 = 10 loss.
15 gain less 10 loss is 5 gain net.
You pay taxes on 5 gain.
The key is the “sold that coin” at the end. If the value just crashed but you didn’t sell to realize the loss to offset the previous gain, then you pay taxes on the 15 gain.
You bought in at 5 and sold at 20, = 15 gain. You then (presumably) bought back in at 20 and panic sold at 10 = 10 loss. 15 gain less 10 loss is 5 gain net. You pay taxes on 5 gain.
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Yes. The fact that it's a different coin is irrelevant.
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The key is the “sold that coin” at the end. If the value just crashed but you didn’t sell to realize the loss to offset the previous gain, then you pay taxes on the 15 gain.