Well there can be two different solutions for this but i will choose the easier one. Obviously one requires a system of equations which obviously will involve a lot of thinking and some good algebra solving in order for them to solve.
As for hit and trial it is very easy, we can just keep adding the distance as we know they are in different directions. 60 + 61= 121. So 121 x 4.5 = 544.5 miles. So after around 4.5 hours they will be 544.5 miles apart if they continue at the same pace.
