Mathematics for IGCSE & O level - Estimation (Section 1 - No. 38)
If a building's height is approximately 20 meters and you want to estimate the height of a tree next to it, what do you need to know?
The exact height of the building.
How many trees there are.
The tree's height is relative to the building.
To know the size of a person.
Explication
Compare to the building's height.