After $t$ seconds have passed, notice that each car has traveled $20t$ meters. Let the remaining horizontal distance be called $x$, and the actual distance be $\ell$:

We want to find the $t$ such that $\ell =500$.

Express $x$ in terms of $t$. Notice that $x,\ell$ and $4\text{m}$ form a right angled triangle so that

After $t$ seconds have passed, notice that each car has traveled $20t$ meters. Let the remaining horizontal distance be called $x$, and the actual distance be $\ell$:

We want to find the $t$ such that $\ell =500$. Notice that

By Pythagoras, we have

We want the first time $\ell =50$, so we need the smaller of the two solution, which is $12.5(1\text{ d.p.})$