We don't know anything about the direction Jack was traveling, so we can't determine his velocity. The best we can do is work with his speed.
Speed = (distance covered) / (time to cover the distance)
Speed = (100 miles) / (2 hours)
Speed = 50 miles/hour
The question has specified a different unit, so we'll need to convert the unit of our answer.
Let's multiply the answer by (1 hour / 3600 seconds). The numerator and denominator of this fraction are both the same quantity, so the value of the fraction is ' 1 ', and multiplying the answer by it won't change the answer in any way except the unit.
Speed = (50 mi / hr) x (1 hr / 3600 sec)
Speed = (50mi · 1hr) / (hr · 3600sec)
Speed = 50 mi / 3600 sec
Speed = 0.01389 mi/sec