Why is it that the number of hours in the day never change yet some days fly by and others drag on? ?

because the season change if you are in summer the days seem longer because the sun stays up longer if you are in the winter the days seem shorter because the sun goes down sooner.

If you're referring to why time flys by so fast or slow depends on what you're doing to keep yourself occupied. I guess that's what you are asking?

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =