On the scale of your paper solar system (1 m = -40 AU), how far would you need to draw the nearest star to Earth if that star was 4 ly away? Show your work.
(Hint: 1 ly = 63241 AU)

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 =