If you can walk 3 miles per hour, how much is that in meters per second?

To convert 3 miles per hour into m/s, first we have to convert miles in to meter and then hour in to second.

As and

Therefore,

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 =