Asample of rock is found to contain 100 grams of a parent isotope. how many grams of the parent isotope will remain after one half-life?

50 grams of the parent isotope will remain after one half-life.

Explanation:

Formula used to calculate remaining amount of an isotope after n half lives:

where,

A = Amount of isotope left after n-half lives

= Initial amount of an isotope

n = number of half lives

Initial amount of an isotope == 100 g

Number of half lives = 1

50 grams of the parent isotope will remain after one half-life.

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 =