the rate of customers who register for postpaid with packages of VND 100,000/month, VND 150000/month and VND 200000/month of a telecommunications service provider respectively is 0.2 ; 0.5; 0.3.A family has 3 phones using this carrier's postpaid plan. call xy, respectively, is the phone number using the 150000 2000 and 200000₫ packages of this carrier among the 3 phones above a .Find the concurrent probability block function of (X, Y) b . Calculate the probability to total charges This family's registered phone is less than 500,000 VND