Given the size and mass of each of the solid cubes x and y, how many
times is the density of cube x greater than the density of cube y?

Since 
density = mass/volume (which is just another word for size)

density of cube X = (cube X's mass) / (cube X's volume)
lets name "(cube X's mass) / (cube X's volume)" as "x"

density of cube Y = (cube Y's mass) / (cube Y's volume)
lets name "(cube Y's mass) / (cube Y's volume)" as "y"

then to calculate how many times is the density of cube X to density of cube Y, then it will be x/y

if calculating how many times greater, then it is (x-y)/y

though the phrase "how many times greater" can also mean "how many times".