Answer:
20 hours later.
Explanation:
If all three buses leave at the same time, they will arrive back in the park in 20 hours. If we want them to arrive at the exact same time, we need to find the least common multiple of all the numbers because the time it takes to get back into the park are different and without using least common multiple they would all arrive at different times. So, the least common multiple is 20 becuase they all have 20 as a multiple. In 20 hours all of the buses will arive back in the park at the same time.
Hope this helps
:)