The number X of fatal traf?c accidents in a certain city during a year may be assumed to be an r.v. distributed as P(l). For the latest year x = 4, whereas for the past several years the average was 10. Test whether it has been an improvement, at level of signi?cance a = 0.01. First, write out the expres- sion for the exact determination of the cut-off point, and secondly, use the CLT for its numerical determination.