# Ch 15 : Probability

**Exercise 15.1**

1. In a cricket match, a batswoman hits a boundary 6 times out of 30 balls she plays. Find the probability that she did not hit a boundary.

**Answer**

Total numbers of balls = 30

Numbers of boundary = 6

Numbers of time she didn’t hit boundary = 30 – 6 = 24

Probability she did not hit a boundary = 24/30 = 4/5

2. 1500 families with 2 children were selected randomly, and the following data were recorded:

Number of girls in a family | 2 | 1 | 0 |

Number of families | 475 | 814 | 211 |

Compute the probability of a family, chosen at random, having

(i) 2 girls (ii) 1 girl (iii) No girl

Also check whether the sum of these probabilities is 1.

**Answer**

Total numbers of families = 1500

(i) Numbers of families having 2 girls = 475

Probability = Numbers of families having 2 girls/Total numbers of families

= 475/1500 = 19/60

(ii) Numbers of families having 1 girls = 814

Probability = Numbers of families having 1 girls/Total numbers of families

= 814/1500 = 407/750

(iii) Numbers of families having 2 girls = 211

Probability = Numbers of families having 0 girls/Total numbers of families

= 211/1500

Sum of the probability = 19/60 + 407/750 + 211/1500

= (475 + 814 + 211)/1500 = 1500/1500 = 1

Yes, the sum of these probabilities is 1.

3. Refer to Example 5, Section 14.4, Chapter 14. Find the probability that a student of the class was born in August.

**Answer**

Total numbers of students = 40

Numbers of students = 6

Required probability = 6/40 = 3/20

4. Three coins are tossed simultaneously 200 times with the following frequencies of different outcomes:

Outcome | 3 heads | 2 heads | 1 head | No head |

Frequency | 23 | 72 | 77 | 28 |

If the three coins are simultaneously tossed again, compute the probability of 2 heads coming up.

**Answer**

Number of times 2 heads come up = 72

Total number of times the coins were tossed = 200

Required probability = 72/200 = 9/25

5. An organisation selected 2400 families at random and surveyed them to determine a relationship between income level and the number of vehicles in a family. The information gathered is listed in the table below:

Monthly income (in ₹) |
Vehicles per family | |||

0 | 1 | 2 | Above 2 | |

Less than 7000 | 10 | 160 | 25 | 0 |

7000-10000 | 0 | 305 | 27 | 2 |

10000-13000 | 1 | 535 | 29 | 1 |

13000-16000 | 2 | 469 | 59 | 25 |

16000 or more | 1 | 579 | 82 | 88 |

(i) earning ₹10000 – 13000 per month and owning exactly 2 vehicles.

(ii) earning ₹16000 or more per month and owning exactly 1 vehicle.

(iii) earning less than ₹7000 per month and does not own any vehicle.

(iv) earning ₹13000 – 16000 per month and owning more than 2 vehicles.

(v) owning not more than 1 vehicle.

**Answer**

Total numbers of families = 2400

(i) Numbers of families earning ₹10000 –13000 per month and owning exactly 2 vehicles = 29

Required probability = 29/2400

(ii) Number of families earning ₹16000 or more per month and owning exactly 1 vehicle = 579

Required probability = 579/2400

(iv) Number of families earning ₹13000-16000 per month and owning more than 2 vehicles = 25

Required probability = 25/2400 = 1/96

(i) Find the probability that a student obtained less than 20% in the mathematics test.

(ii) Find the probability that a student obtained marks 60 or above.

Marks | Number of students |

0 – 20 | 7 |

20 – 30 | 10 |

30 – 40 | 10 |

40 – 50 | 20 |

50 – 60 | 20 |

60 – 70 | 15 |

70 – above | 8 |

Total | 90 |

**Answer**Total numbers of students = 90

(i) Numbers of students obtained less than 20% in the mathematics test = 7

Required probability = 7/90

(ii) Numbers of student obtained marks 60 or above = 15+8 = 23

Required probability = 23/90

7. To know the opinion of the students about the subject statistics, a survey of 200 students was conducted. The data is recorded in the following table.

Opinion | Number of students |

like | 135 |

dislike | 65 |

Find the probability that a student chosen at random

(i) likes statistics, (ii) does not like it.

**Answer**Total numbers of students = 135 + 65 = 200

(i) Numbers of students who like statistics = 135

Required probability = 135/200 = 27/40

(ii) Numbers of students who does not like statistics = 65

Required probability = 65/200 = 13/40

8. Refer to Q.2, Exercise 14.2. What is the empirical probability that an engineer lives:

(i) less than 7 km from her place of work?

(ii) more than or equal to 7 km from her place of work?

(iii) within 1/2 km from her place of work?

**Answer**

The distance (in km) of 40 engineers from their residence to their place of work were found as follows:

5 3 10 20 25 11 13 7 12 31 19 10 12 17 18 11 3 2 17 16 2 7 9 7 8 3 5 12 15 18 3 12 14 2 9 6 15 15 7 6 12

Total numbers of engineers = 40

(i) Numbers of engineers living less than 7 km from her place of work = 9

Required probability = 9/40

(ii) Numbers of engineers living less than 7 km from her place of work = 40 – 9 = 31

Required probability = 31/40

(iii) Numbers of engineers living less than 7 km from her place of work = 0

Required probability = 0/40 = 0

Page No: 285

11. Eleven bags of wheat flour, each marked 5 kg, actually contained the following weights of flour (in kg):

4.97 5.05 5.08 5.03 5.00 5.06 5.08 4.98 5.04 5.07 5.00

Find the probability that any of these bags chosen at random contains more than 5 kg of flour.

**Answer**

Total numbers of bags = 11

Numbers of bags containing more than 5 kg of flour = 7

Required probability = 7/11

12. In Q.5, Exercise 14.2, you were asked to prepare a frequency distribution table, regarding the concentration of sulphur dioxide in the air in parts per million of a certain city for 30 days. Using this table, find the probability of the concentration of sulphur dioxide in the interval 0.12-0.16 on any of these days.

The data obtained for 30 days is as follows:

0.03 0.08 0.08 0.09 0.04 0.17 0.16 0.05 0.02 0.06 0.18 0.20 0.11 0.08 0.12 0.13 0.22 0.07 0.08 0.01 0.10 0.06 0.09 0.18 0.11 0.07 0.05 0.07 0.01 0.04

**Answer**

Total numbers of days data recorded = 30 days

Numbers of days in which sulphur dioxide in the interval 0.12-0.16 = 2

Required probability = 2/30 = 1/15

13. In Q.1, Exercise 14.2, you were asked to prepare a frequency distribution table regarding the blood groups of 30 students of a class. Use this table to determine the probability that a student of this class, selected at random, has blood group AB.

The blood groups of 30 students of Class VIII are recorded as follows:

A, B, O, O, AB, O, A, O, B, A, O, B, A, O, O, A, AB, O, A, A, O, O, AB, B, A, O, B, A, B, O.

**Answer**

Total numbers of students = 30

Numbers of students having blood group AB = 3

Required probability = 3/30 = 1/10