1 Why is the unit of pressure caled a derved unit?

Pressure = (force) divided by (area)

But force and area are both derived units themselves.

-- Force = (mass) times (acceleration)

But acceleration is a derived unit itself.

Acceleration = (length) divided by (time squared)

So force = (mass) · (length / time squared)

-- Area = (length squared)

So pressure = (mass · length / time-squared) / (length-squared)

Pressure = (mass) / (length · time-squared)

That sure looks to me like it's derived from a bunch of other stuff.

it takes about 30 days for it to break down

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.