Answer:
t=4.86s
Explanation:
To find the wavelength you use the following formula:

v: speed of sound = 343m/s
f: frequency = 400Hz
λ: wavelength of the sound
By doing λ the subject of the formula and replacing the values of f and v you obtain:

Now, to calculate the time that sound takes to reach the last row you use:

t: time
d: distance to the last row = 1947m

hence, the time is 4.86s
milligrams
Explanation:
The best unit to measure the mass of a grain of salt is the milligram.
This is about; a thousandth of a gram.
A grain of salt is a very small particle size that can still be visible with the eye.
It has a very low and small mass.
For substances like this, we use the milligram:
1000milligram = 1g
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