A bullet train leaves Kyoto for

Hello Vincen.

Let's take a look at your question.

After the first 10 minutes, the bullet train will have traveled $$240\cdot\left(\frac{10}{60}\right)\ =\ 40\ \ \ miles.$$ Therefore, at 12:10, the distance between the bullet train and the other train is 300 - 40 = 260 miles.

The combined rate of the two trains is 240+160=400 miles per hour. Now we have to divide Distance/Rate, to find total time each will travel.

Hence, $$t=\frac{d}{v}\ \Leftrightarrow\ t=\frac{260\ m}{400\ \frac{m}{h}}=\ \frac{39h}{60}\ \\=\ \frac{39h}{60}\cdot\frac{60\min}{1\ h}=39\ \min\ \Rightarrow\ \ 39\ \text{minutes to meet each other}.$$ We have to notice that x = 39 minutes for the trains to pass each other AFTER 12:10.

Therefore the trains must have passed each other at 12:49 pm.

So, the correct answer is B.

I hope this explanation may help you.

I'm available if you'd like a follow-up.

Regards.