In a game the player wins if he rolls a 6 on a number cube. if the number cube is rolled 18 times, then what is a reasonable prediction for the number of unsuccessful rolls?

On a cube, you have 6 faces, therefore you have 1 chance to get 6 and 5 chances not to get 6.

Therefore, the overall probability not to get 6 is 5/6 = 0.8333

If you roll the cube 18 times, the prediction of unsuccessful rolls will be:
18 × 0.8333 = 15

Therefore, you expect to have 15 unsuccessful rolls over 18 total rolls.