A coffee stand operates with five baristas, each of whom serve a separate line of customers. Customers arrive at each line in separate Poisson streams at the rate of $18/$hour$.$ The baristas are able to provide exponentially$-$distributed service at the rate of $23/$hour$.$ Two options for improvement of the coffee stand are being considered:

Design A: Merge the five arrival streams into a single Poisson stream with rate $90/$hour$,$ and pool the five existing baristas to serve it $($they each continue to provide exponentially$-$distributed service at rate $23/$hour$).$

Design B: Hire a $6$th barista, and have the baristas $($who each provide exponentially$-$distributed service at the rate of $23/$hour$)$ each serve a separate Poisson stream of customers who arrive at the rate of $15/$hour$.$

Which design produces the lowest average time in system?