Question

twenty balls are serially numbered and placed in a bag. the probability that a ball drawn is a multiple of 3 or 5 is

Answer 1

Multiple of 3 in 20 = 6 (3,6,9,12,15,18)
Multiple of 5 in 20 = 4 (5,10,15,20)
Total Multiple of 3 and 5 = 6+4-1(15 is common)=9
Answer - 20C9

Answer 2

The sample space is given as
S={1,2,3,4......20}
N(S)=20
X=Probabililty that ball drawn is a multiple pf 3 or 5
X={1,3,5,6,9,10,12,15,18,20}
N(X)=10
P(X)=N(X)/P(X)
=10/20
=0.2