From the basic knowledge of ratio:
cars : trucks
[tex]\begin{gathered} \frac{5}{4}\text{ = }\frac{x}{100} \\ \text{cross}-\text{ multiply,} \\ 4\text{ }\times\text{ x = 5 x 100} \\ 4x\text{ = 500} \\ \text{Divide both sides, we have:} \\ x\text{ =500/4} \\ x\text{ = 125} \end{gathered}[/tex]