Twenty sheets of aluminium alloy were examined for surface flaws.The frequency of the number of sheets with a given number of flaws per sheet was as follows

Question

Number of flaws	Frequency
0	4
1	3
2	5
3	2
4	4
5	1
6	1

What is the probability of finding a sheet chosen at random Which contain 3 or more surface flaws? 

Answer 1

4/20+3/20+5/20=12/20 sheets have less than 3 flaws, so 8 have at least 3 flaws, so the probability is 8/20=4/10, 0.4 or 40%. (Or simply add 2+4+1+1 for sheets with 3 or more flaws.)