Page 272 - ICSE Math 8

Number of not rotten eggs = 20 – 8 = 12
Number of not rotteneggs 12 3
\ P(Not a rotten egg) = = =
Total numberofeggs 20 5
Alternate Method
Let A be the event of taking out a rotten egg.
Number of rotteneggs 8 2
P(A) = = =
Totaleggs 20 5
2 3
\ P (Egg is not rotten) = P A () = 1 – P(A) = 1 – =
5 5
Example 8: From a well-shuffled deck of 52 cards, one card is drawn at random. Find the probability of
getting
(i) an ace. (ii) a 10 of black cards. (iii) a face card.
Solution: Total number of all cards = 52
Total number of all possible outcomes = 52
(i) Number of ace cards = 4
4 1
∴ P(getting an ace) = =
52 13
(ii) Number of 10s of black cards = 2
2 1
∴ P(getting a 10 of black cards) = 52 = 26
(iii) Number of face cards = 12
12 3
∴ P(getting a face card) = =
52 13
Example 9: A survey of 150 families shows the
following results. Out of these families one Number of girls in the family 0 1 2
is chosen at random. What is the probability Number of families 30 115 5
that the chosen family has 2 girls?
Solution: Let E be the event of picking a family with 2 girls.
Total number of families = 150
Number of families with 2 girls = 5
Number of families with 2girls 5 1
\ P (E) = = =
Total numberoffamilies 150 30

Example 10: An urn contains 4 white, 3 black and 5 red marbles. A marble is drawn at random. What is the
probability that it is:

(a) red? (b) white? (c) not black?
Solution: Total number of marbles = 4 + 3 + 5 = 12
Number of redmarbles 5
(a) P (Red marble) = =
Total numberofmarbles 12
Number of white marbles 4 1
(b) P (White marble) = = =
Total numberofmarbles 12 3

Number of Black marbles
(c) P (Not Black marble) = 1 – P (Black marble) = 1 –
Total numberofmarbles
3 1 41− 3
= 1 – = 1 – = =
12 4 4 4

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