The rate of the height of an object to the length of its shadow must be the same if both objects are at the same place at the same time.
Let h be the height of the building. Since it casts a 90 meter long shadow while a 1.5 meter tall tenant casts a 0.75 meter shadow, then:
[tex]\begin{gathered} \frac{h}{90}=\frac{1.5}{0.75} \\ \Rightarrow\frac{h}{90}=2 \\ \Rightarrow h=2\times90 \\ \Rightarrow h=180 \end{gathered}[/tex]Therefore, the apartment building is 180 meters tall.
