Let T = time Dave drove, then T+1 = time Alan drove.
65T = distance Dave drove and 55(T+1) = distance Alan drove
65T + 55(T+1) = total distance driven by both = 250
120T + 55 = 250
120T = 195 => T = 1.625
1.625 hours after Dave left, they will run out of cell phone coverage and you should use the answer to check that this is so.