What is likely to happen if a balloon that was blown up at a low altitude is brought to a high altitude? why would this occur?

If the pressure is lowered, than the volume must increase. Therefore, when the pressure goes down, the balloons get larger and larger. They may even pop, depending on how full they were to start with. But if they don't pop, when the pressure went back up (you climbed back down to the classroom)... then the balloon would go back to it's normal size.

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 =

god i hope this !