It has to due with numbers so I would say the last one!
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


Given
Three 7 ohm resistor are in series.
The battery is V=10V
To find
The equivalent resistance
Explanation
When the resistance are in series then the resistance are added to find its equivalent.
Thus the equivalent resistance is:

Conclusion
The equivalent resistance is 21 ohm
Answer:
8.94*10^22 kg
Explanation:
Given that
Mass of Lo, M = ?
Radius of Lo, r = 1.82*10^6 m
Acceleration on Lo, g = 1.80 m/s²
Gravitational constant, G = 6.67*10^-11
Using the formula
g = GM/r²
Solution is attached below
Answer is 8.94*10^22 kg