Stardust the unicorn, covered a distance of 50 miles on his first trip to the forest. on a later trip he travelled 300 miles while going three times as fast. his new time compared with the old time was:

twice as long

Explanation:

If Stardust managed to cover 50 miles on his first trip to the forest, and later managed to cover 300 miles by going 3 times faster, then we can easily come to the result as to how much more time Stardust needed comparably between the two trips, just by using these three numbers.

If Stardust was going 3 times as fast on the second trip, then we need to multiple that number with the number of miles of the first trip:

50 x 3 = 150

This means that Stardust managed to cover 150 miles on the second trip in the same amount of time as the 50 miles in the first trip. In order to see how much more time Stardust needed to cover the 300 miles, we just need to divide the total amount of miles, with the 150 miles of the 3 times faster speed:

300 / 150 = 2

So we get a result of 2, meaning that Stardust needed twice more time to cover the 300 miles, than what he needed to cover the 50 miles of the first trip.