I believe Intangibility is the answer! :P I hope this helps!
Use the concept of beat frequency to find the applicable final freqeuncy for 20Hz beat frequency.
Beat can be defined as 'the interference pattern between two sounds of slightly different frequencies0
The expression for beat frequency is given as

Where,
Final frequency
Initial frequency
The beat frequency for us is 25Hz and the initial frequency is 240Hz, then

Being an absolute value, two values are possible, both in addition and subtraction:

The two possible values are


During that period of time, the bird's displacement was 4 km east. So its velocity was (4km east)/(11hrs). That's 0.36 km/hour east. (rounded)
The relationship between inductance and frequency can be clearly described using the following equation of inductive reactance:
Xl = 2*pi*f*L ; simplifying:
L = Xl / 2*pi*f
Therefore, as what we saw, inductance and frequency are inversely proportional. To add up, when inductance increases the frequency would decrease.
Nonmetals will readily form covalent bonds with other nonmetals in order to obtain stability, and can form anywhere between one to three covalent bonds with other nonmetals depending on how many valence electrons they posses.