Computing mixed strategy Nash equilibria?

Consider the following social problem.^5 A pedestrian is hit by a car and lies injured on the road. There are n people in the vicinity of the accident. The injured pedestrian requires immediate medical attention, which will be forthcoming if at least one of the n people calls for help. Simultaneously and independently, each of the n bystanders decides whether or not to call for help (by dialing 911 on a cell phone or pay phone). Each bystander obtains upsilon units of utility if someone (anyone) calls for help. Those who call for help pay a personal cost of c. That is, if person i calls for help, then he obtains the payoff upsilon - c; if person i does not call but at least one other person calls, then person i gets upsilon; finally, if none of the n people calls for help, then person i obtains zero. Assume upsilon > c. Consider the following social problem.^5 A pedestrian is hit by a car and lies injured on the road. There are n people in the vicinity of the accident. The injured pedestrian requires immediate medical attention, which will be forthcoming if at least one of the n people calls for help. Simultaneously and independently, each of the n bystanders decides whether or not to call for help (by dialing 911 on a cell phone or pay phone). Each bystander obtains upsilon units of utility if someone (anyone) calls for help. Those who call for help pay a personal cost of c. That is, if person i calls for help, then he obtains the payoff upsilon - c; if person i does not call but at least one other person calls, then person i gets upsilon; finally, if none of the n people calls for help, then person i obtains zero. Assume upsilon > c