Answer:

Explanation:
From the question we are told that:
Frequency of 3rd harmonics 
Frequency of 5th harmonics 
Generally the equation for Wavelength at 3rd Harmonics is mathematically given by

Therefore

Generally the equation for Wavelength at 1st Harmonics is mathematically given by

Therefore

Generally the equation for the frequency of the first harmonic is mathematically given by



Answer:
Explanation:
Force - frictional Force = m * a
20N - 5N = 5 * a
15N = 5*a
15N / 5 = a
a = 3 m/s^2
You should note a couple of things.
1. The frictional Force always opposes the direction of motion. In this case it is minus and the 20 N is plus. It makes the acceleration less. If the friction force was not there then 20 N = m * a
20N = 5 kg * a
a = 20/5
a = 4 m/s^2.