Page 353 - Start Up Mathematics_8 (Non CCE)

Alternate Method
Total number of eggs in the box = 20

Number of defective eggs = 8
Let A be the event of taking out a defective egg.
Number of defectiveeggs 8 2
P(A) = = =
Totaleggs 20 5
\ P (Egg is not defective) = PA () = 1 – P(A)
2 3
= 1 – =
5 5

Example 10: A survey of 150 families shows the following results.
Out of these families one is chosen at Number of girls in the family 0 1 2
random. What is the probability that the 30 115 5
chosen family has 2 girls? Number of families

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 11: 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: Number of white marbles = 4
Number of black marbles = 3
Number of red marbles = 5
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

(c) P (Not black marble) = 1 – P (Black marble)
Number of black marbles
= 1 –
Total numberofmarbles

3 1 41- 3
= 1 – = 1 – = =
12 4 4 4

EXERCISE

1. Two fair coins are tossed 200 times. Two heads are obtained 120 times. Now, if the coin is tossed
at random, what is the probability of getting 2 heads?
2. Two fair coins are tossed simultaneously. Find the probability of getting:
(a) 2 tails. (b) no head. (c) no tail.
(d) same face on each coin. (e) different faces on the coins.

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