A small plane travelled 1506 km in 9.5 h. During the first part of the trip, the plane travelled at an average speed of 150 km/h. During the rest of the trip, the plane travelled at an average speed of 180 km/h. For how long did the plane travel at 150 km/h?

6

Step-by-step explanation:

This is a case of combination since the order does not matter, it only matters that there must be a marble in each one inside, therefore we must apply its formula.

xCn = n! / x! * (n - x)!

n, in this case it would be 4 because we must subtract 2 from the total 6, since there must already be one inside in each display case.

x = 2, since it is the number of showcases.

Replacing:

 2C4 = 4! / 2! * (4 - 2)! = 6

Which means that you can in six different ways.