To develop this problem we will apply the concepts related to the Doppler effect. The frequency of sound perceive by observer changes from source emitting the sound. The frequency received by observer
is more than the frequency emitted by the source. The expression to find the frequency received by the person is,

= Frequency of the source
= Speed of sound
= Speed of source
The velocity of the ambulance is


Replacing at the expression to frequency of observer we have,


Therefore the frequency receive by observer is 878Hz
One of the fundamental pillars to solve this problem is the use of thermodynamic tables to be able to find the values of the specific volume of saturated liquid and evaporation. We will be guided by the table B.7.1 'Saturated Methane' from which we will obtain the properties of this gas at the given temperature. Later considering the isobaric process we will calculate with that volume the properties in state two. Finally we will calculate the times through the differences of the temperatures and reasons of change of heat.
Table B.7.1: Saturated Methane




Calculate the specific volume of the methane at state 1



Assume the tank is rigid, specific volume remains constant


Now from the same table we can obtain the properties,
At 


We can calculate the time taken for the methane to become a single phase

Here
Initial temperature of Methane
Warming rate
Replacing



Therefore the time taken for the methane to become a single phase is 5hr
The force of attraction between the two particles will remain the same, because when mass is doubled, force of attraction is doubled. However, when distance between their centers is doubled, then force of attraction is halved. As such double and half cancel out each other and force of attraction remains the same.
I see the light moving exactly at speed equal to c.
In fact, the second postulate of special relativity states that:
"The speed of light in free space has the same value c<span> in all inertial frames of reference."
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The problem says that I am moving at speed 2/3 c, so my motion is a uniform motion (constant speed). This means I am in an inertial frame of reference, so the speed of light in this frame must be equal to c.