Explain why
used
why composite samples​

samples are combined, thoroughly homogenized, and treated as a single sample. Composite sampling can improve spatial or temporal coverage of an area without increasing sample number. Appropriateness of composite sampling is dependent upon the sampling objectives and the site characteristics.

Explanation:

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 =