homework help: how do I solve hypothesis testing without standard deviation?

Q7. Law enforcement officers in a city have claimed that over 40% of all robberies committed by those who have a history of drug abuse and/or alcohol abuse. The Journal of Research in Crime and Delinquency recently reported that in a random sample of 96 robberies, 40 were found to be committed by people who had drugs and/or alcohol abuse problems. Does this data substantiate the law enforcement officers’ claim at a 5% level of significance (i.e., alpha=0.05)? [8 points]

since the 96 robberies is more than 30, I would use the z test. the z test requires a standard deviation. am i looking at this wrong?