By definition, the number
is the ratio between the circumference and the diameter:

This implies that

And so your circumference is 
Answer:
2 2/19
Step-by-step explanation:
5 cases in 2 3/8 hours
The unit rate of cases per hour is calculated by dividing the number of cases by the number of hours.
5/(2 3/8) = 5/(19/8) = 5 * 8/19 = 40/19 = 2 2/19
Let x = hours to meet
train one has a 2 hour head start or 210 km head start
210 + 105x = 135x
30x = 210
x = 7 hours to meet