Train a arrives at central station on the hour and every 12 minutes. train b arrives on the hour and every 15 minutes. when do both trains arrive at the same time?
a) on the hour and 30 minutes past the hour
b) on the hour and 15 minutes to the hour
c) on the hour and 27 minutes past the hour
d) on the hour
explain why you chose the answer and how you achieved it.