Let the total number of offices in the building be x.
We can represent the information provided in the question mathematically as:
[tex]30\text{ \% of x = 42}[/tex]Solving for x:
[tex]\begin{gathered} \frac{30}{100}\text{ }\times\text{ x = 42} \\ \frac{30x}{100}\text{ = 42} \\ \end{gathered}[/tex]Cross-Multiply:
[tex]\begin{gathered} 30x\text{ = 4200} \\ \text{Divide both sides by 30} \\ \frac{30x}{30}\text{ = }\frac{4200}{30} \\ x\text{ = 140} \end{gathered}[/tex]Hence, there are 140 offices in the building.
Answer: 140 offices