An Atwood's machine is built with a massless and frictionless pulley with a mass of 1 kilogram on one side and a mass of 4 kilograms on the other side. Assuming that the acceleration due to gravity is 10 meters per second squared, which is the speed of the masses in meters per second three seconds after being released?

W) 5

X) 6

Y) 15

Z) 18
Z) 18