Saul Goodman loves a quick bite at Los Pollos Hermanos. Saul knows from Gustavo that Los Pollos receives a new customer every $3$ minutes on average $($coefficient of variation $=1)$ and on average it takes $2$ minutes $($standard deviation of this time is $2$ mins$)$ for the food to be prepared and served. The counter can serve at most one person at a time and the first arriving customer is served first. What is the inter$-$arrival time in minutes?