Answer
a. 18000 integers are divisible by 5
b. 1/5 = 20% probability of random 5-digit integer being chosen that is divisible by 5.
Work Step by Step
A1 = integer ending with 0, A2 = integer ending with 5
a. N(A1) = 1*9*10*10*10 = 9000
N(A2) = 1*9*10*10*10 = 9000
N(A1 ⋃ A2) = 9000 + 9000 = 18000 integers are divisible by 5
b. 18000/((99999 – 10000) + 1)= 18/90 = 3*6/(3*30) = 1/5 = 20% probability of random 5-digit integer being chosen that is divisible by 5.