$($a$)$ Given that each user typically makes a call attempt every $10$minutes on the average. What should the call arrival rate $($call attempts per unit time$)?$

$($b$)$ Find the mean m if Poison distribution p$($x$=1)+$p$($x$=2)=$p$($x$=3).$

P$($X$=$i$)$ where the random variable X follows POISSON DISTRIBUTION.