When 6 rolls are used, there are 18 - 6 = 12 remaninig rolls. 12 is the total of rolls remaining, then, we need to find the fraction of 18 that corresponds to 12, being 18 the total. For that, we just need to divide them:
We can now simplify. We can divide both numerator and denominator by 6, since 6*2 = 12 and 6*3 = 18:
Then, 12 is two thirds of 18. We can say that after 6 of the 18 rolls are used, the remaining amount of rolls is two thirds of the previous amount of rolls.