A super market has $8$ checkout lanes. It wants to decide how many lanes should be labeled as express lanes, which will serve customers with fewer than $10$ items. Customer arrival to express lanes is exponentially distributed with rate $20$ customers per hour, whereas the arrival to regular lanes is exponentilly distributed with rate $10$ customers per hour. The average time spent in express lanes is $10$ mintues per customer, and the average time spent in regular lanes is $20$ minutes.

How many of the lanes should be dedicated to express customers so that both queues are stable?