How can I calculate the time two people will meet if they are paddling towards each other on a lake? Here's the question I'm having trouble with. Ken and Kara are 30 miles apart on a calm lake paddling towards each other. Ken paddles at 4 mph, while Kara paddles at 7 mph. HOw long will it take them to meet?
I suppose that you know how to find variable $t$ (time) from system below :

$$\begin{cases}
 x + y = 30 \\
 x=4\cdot t \\
 y=7 \cdot t
\end{cases}$$
You can use relative velocities, the difference between their velocities is 3mph, and the distance is 30 miles, so it would take 10 hours for them to cover the same distance.