We know that in physics the momentum of an object is defined as the product of mass by the velocity of the object.
That is to say:

Where P is the momentum =
kilograms meters/second
m is the mass
v is the object velocity = 30 m/s
In this problem we have the momentum of the roller coaster and its speed, but we want to find its mass. Then we divide the momentum between speed.



The mass is 833.33 kg