A train is leaving in 11 minutes and you are one mile from the station. Assuming that you can walk at 4mph and run at 8mph, how much time can you afford to walk before you start running?

At first I thougt it was 3 min and 30 seconds

because walk 1 mile / 15 min

run 1 mile / 7.5 min

but now I think I could actually walk more because if I walk for 3min 30 seconds I would have already traveled 1/5 of the mile.

I am stuck from here.

Vicky.