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Your BIL likely has an intellectual impairment to get those benefits. Learning disabilities are not typically severe enough to disable a person from working. An intellectual impairment is a much broader term that essentially indicates the person’s IQ is significantly lower than average, and this very often is disabling enough to prevent a person from working.
Yes, because its calculation relies on the local speed of sound, which is varies based on several factors, including altitude.
https://en.wikipedia.org/wiki/Mach_number