<h2>Time required to complete the round trip 
where x is average velocity on the outgoing trip.</h2>
Step-by-step explanation:
Let average velocity of outgoing trip = x mph
The average velocity on the return trip is 15 miles per hour slower than the average velocity on the outgoing trip.
Average velocity of return trip = (x-15) mph
Distance to vacation place = 420 miles
Distance to vacation place = Time for outgoing trip x average velocity of outgoing trip
Distance to vacation place = Time for return trip x average velocity of return trip
We have total time T = t₁ + t₂
That is
Time required to complete the round trip where x is average velocity on the outgoing trip.