In order to start a fire, a camper turns a lens toward the sun to focus its rays on a piece of wood. the lens has a 12 cm focal length. use a ray diagram of the lens and the incoming light rays to find the distance from the lens at which the wood should be placed for the best effect.

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 =