Q> 60% of the buses at my local bus stop are on route 39. If the routes of different buses are independent of each other, what is the probability that at least 4 buses that are not route 39 arrive before the 3rd route 39 bus ?

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I really don't understand these types of questions. The wording throws me for a loop

I know that it is probably neg binom but even with the formula I draw a blank for how to organize everything.

'at least' ? - is this the reason that this problem can't be done with only 1 application of the neg binom formula ?

'arrive before' ? - what if it were 'arrive on' or 'arrive after' ? how would these effect the total of successes+failures 'n' ?