How Many Buses Departed While Jill Waited for Jack?

[recon]

Every day, Jack and Jill agree to meet at a certain time at the nearby bus interchange, where buses depart at equal periods of time. Once, Jill came 15 minutes later and Jack saw 6 buses depart. On a second occasion, Jill came 26 minutes later, and Jack saw 8 buses depart. On another occasion, Jack came 43 minutes later than Jill. How many buses departed the interchange while Jill was awaiting Jack?

Find all possible answers.

I'm quite confused by this question at the moment. I've got to run now, and I may try and post my attempt a bit later, but I doubt I'm heading in the right direction.

 

[SpaceTiger]

Explore the limits of your system. What are the maximum and minimum possible periods between buses for each of the first two criteria? If I have an interval of time that's just over two bus periods long, how many buses can I fit inside it? If it's just under two bus periods long, how few buses can be fit inside? What does this tell you about the limits on the period?