Two planes fly in opposite directions. one travels 450 mi/h and the other 550 mi/h. how long will it take before they are 4,000 mi apart?

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. 450 + 550 = 1000. So 1000 x 4 = 4000 miles. So after around 4 hours they will be 4000 miles apart if they continue at the same pace.