Acar travels 20 km north. it then turns around and travels 30 km south. what is the car's displacement?

10 km south

Explanation:

The two displacements of the car are vectors, so they have a magnitude and a direction.

The first displacement is 20 km north.

The second displacement is 30 km south.

Since the two displacements are along the same direction (north-south), we can calculate their resultant simply adding them, but taking one of the two with a negative sign (because they are opposite). If we take south as positive direction, and north as negative direction, the resultant displacement becomes

and since it has positive sign, the direction is south.

there is no correct list among the choices you've provided.

the list might look something like this:

-- baby human

-- piccolo

-- flute

-- soprano singer

-- trumpet

-- tenor singer

-- trombone

-- baritone singer

-- baritone horn

-- bass singer

-- tuba

since i had to make up the list myself, i made it easy on myself, and i didn't try to shuffle the string instruments and woodwinds into it too.

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 =