The event of pedestrians walking is a and the event of cars pulled over for speeding is b. if these events are independent events, and p(a)=0.27, and p(b)=0.71, what is p(b|a)?

Acar started driving from city a to city b, distance between which is 620 miles, at a rate of 60 mph. two hours later a truck left city b and started moving towards city a at a rate of 40 mph. how many hours after the truck's departure will the two meet?