Sound intensity is inversely proportional to the square of the distance between the source and the receiver.
That is
I = k/r^2
where
k = constant
r = radius
When r=1, the intensity is I₁ = k/1 = k
When r=3, the intensity I₂ = k/3² = k/9
Therefore
I₂ = I₁ /9
In decibels,
I = 10 log₁₀(I/I₀)
where I₀ = reference intensity
When r=1,
10 log₁₀ (I₁/I₀) = 270
When r =3,
10 log₁₀ (I₂/I₀) = 10 log₁₀ [(I₂/I₁)*(I₁/I₀)]
= 10 log₁₀ [(1/9)*(I₁/I₀)]
= 10 log₁₀(1/9) + 270
= 260.5
Answer: 260.5 dB (nearest tenth)
All you need to do is to convert the velocity to meters per hour. Then multiply by 0.4 hours. I guess the answer was rounded up.
Beryl is shaped like the hexagonal crystal system
