Hi all. Ive been trying to get this for weeks now. i have an mm1 Que with 4 services provided. values are

mean service time probability service is required
4.78 0.39
4.66 0.26
5.55 0.18
6.81 0.17

the lambda is 11.75 per hour and these values are in minutes so i work out the mu is 11.4667? but this gives me a negative value every time i plug it into the formula. Any advice??

this is the formula for no one in the Que

Po= 1-lambda/mu