A driver is on a highway that has provided an odometer test zone in which signs mark
every
kilometre for 25 km. He checks his
watch and sees that it is 1:16 P. M. when he
passes the 5.0 km marker. When he passes
the 25 km marker, his watch reads 1:31 P. M.
What was his speed for that section of the
test zone?

3.2x10^7

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 =