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**Prove It** Setting up the system is easy.

For the first, it starts at the origin, so just remember, $\displaystyle d = tv$.

In this case, the first equation is $\displaystyle d = 30t$.

In the second, it leaves 15 minutes later, i.e. 0.25 of an hour later.

So point 1 is $\displaystyle (t, d) = (0.25, 0)$.

If it travels at 40miles/hour, then $\displaystyle m = 40$.

Plug them all into the equation $\displaystyle y = mx + c$ to get c. (in this case, y = d = 0, x = t= 0.25, m = v = 40).

You should find that $\displaystyle c = -10$

So the second equation is $\displaystyle d = 40t - 10$.

Therefore the system of equations is

$\displaystyle d = 30t$

$\displaystyle d = 40t - 10$.

Can you solve them to see where they meet?