Answer:
259 Hz or 269 Hz
Explanation:
Beat: This is the phenomenon obtained when two notes of nearly equal frequency are sounded together. The S.I unit of beat is Hertz (Hz).
From the question,
Beat = f₂-f₁................ Equation 1
Note: The frequency of the other instrument is either f₁ or f₂.
If the unknown instrument's frequency is f₁,
Then,
f₁ = f₂-beat............ equation 2
Given: f₂ = 264 Hz, Beat = 5 Hz
Substitute into equation 2
f₁ = 264-5
f₁ = 259 Hz.
But if the unknown frequency is f₂,
Then,
f₂ = f₁+Beat................. Equation 3
f₂ = 264+5
f₂ = 269 Hz.
Hence the beat could be 259 Hz or 269 Hz
Step by step solution :
standard deviation is given by :

where,
is standard deviation
is mean of given data
n is number of observations
From the above data, 
Now, if
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If
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if
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If
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If
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so, 



No, Joe's value does not agree with the accepted value of 25.9 seconds. This shows a lots of errors.
In a mixture, there will be a solute and solvent. The hydrogen and oxygen are still two different atoms that just mixed together. But in a compound, the hydrogen and oxygen have a bond, making a new and bigger molecule. A mixture can easily be separated by physical means but a compound isn't.
It is based on their state like population, literacy and poverty it varies from state to state, country to country