When the net force of opposite forces is zero, the forces are
a) not even
b) unequal
c) unbalanced
d) balanced

54 degrees incident angle = reflected angle

angle is from normal turned to line of ray

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 =