Explanation

When Bilal moves away from a stationary organ producing a sound of frequency 'f' with a speed equal to 0.5 times the speed of sound, he will hear a sound of lower frequency.

This phenomenon is known as the Doppler effect.

To calculate the frequency of the sound Bilal hears, we can use the following formula:

f' = f * (v + vL) / (v + vS)

Where:

- f' is the frequency of the sound heard by Bilal

- f is the frequency of the sound produced by the stationary organ

- v is the speed of sound

- vL is the speed of Bilal

- vS is the speed of the source (in this case, the stationary organ)

Since Bilal is moving away from the source, vS is negative.

We substitute the values into the formula:

f' = f * (v - 0.5v) / (v - (-v))

Simplifying the equation:

f' = f * (0.5v) / (2v)

f' = f * 0.5 / 2

f' = f * 0.25