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True, frozen water at -20 degree celsius has the same BTU like salted frozen water at -20 degrees. But the salted water should reach the -20 degrees faster since it's longer in liquid form (liquid water has an higher termal exchange than frozen, AFAIK)
For making ice cream, you salt the water so it can drop below 0 degree celsius while still staying liquid and keep it's higher termal exchange rate...
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