can somebody help me answer this question:

By law, a box of cereal labeled as containing 20 ounces must contain at least 20 ounces of cereal. The machine filling the boxes produces a distribution of fill weights with a mean equal to the setting on the machine and with a standard deviation equal to 0.03 ounce. To ensure that most of the boxes contain at least 20 ounces, the machine is set so that the mean fill per box is 20.09 ounces. Assuming nothing is known about the shape of the distribution, what can be said about the proportion of cereal boxes that contain less than 20 ounces.

I am having a hard time even figuring out what formula to use...I have tried by figuring out the Z score and finding its value by subtracting mean from the x (20 ounces), but that does not return the correct result, which appears to be The proportion is at most 11%.
I hope you can help.

Thanks.