Grocery 1" at Lugano operates six checkout counters, each serving its own waiting line

Grocery 1" at Lugano operates six checkout counters, each serving its own waiting line. On average, 90 customers per hour are arriving to "G1" following Poisson distribution. The service time is 3 minutes per customer following exponential distribution. 
The waiting cost of a customer in the system is $30 per hour. G1 is employing one worker at each checkout counter at an hourly wage of $20. 
Suppose all customers are pooled into a single queue, how many checkout counters would be optimum?