Your question is not complete, but I will assume the question is, at what time does Gregory catch up with Efron, and that the 30 is 30 minutes, and that they both started from the same place, and both were both headed in the same direction...
The basic distance formula is rt = d, or rate × time = distance.
For Efron this would be: 4mi/hr × t = d
For Gregory 8.5mi/hr x (t - 30min.) = d
(t - 30min.) because Gregory started 30 minutes after Efron did.
However, since my rate is measured in hours, I have convert one of them.
30 min. = ½ hour = 0.5 hours.
Therefore; 8.5mi/hr x (t - 0.5hr) = d.
Since Efron and Greogory both start at the same place and end up meeting each other en route, the distance they have covered is the same so Efron's "d" = Greogory's "d". Therefore:
(4mi/hr)(t) = (8.5mi/hr)(t - 0.5hr)
4tmi/hr= (8.5mi/hr)(t) - (8.5mi/hr)(0.5hr)
4tmi/hr = 8.5tmi/hr - 4.25mi
-4.5tmi/hr = -4.25mi
t = (-4.25mi)/(-4.5mi/hr)
t = .944444444hr
t = 56 min. & 40 sec.