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The following are the marks obtained by 70 boys in a class test:

Marks | No. of boys |

30-40 | 10 |

40-50 | 12 |

50-60 | 14 |

60-70 | 12 |

70-80 | 9 |

80-90 | 7 |

90-100 | 6 |

Calculate the mean by :

Step - deviation method

Attempt this question on a graph paper. The table shows the distribution of marks gained by a group of 400 students in an examination.

Marks (Less than ) |
10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 |

No.of student | 5 | 10 | 30 | 60 | 105 | 180 | 270 | 355 | 390 | 400 |

Using scaie of 2cm to represent 10 marks and 2 cm to represent 50 student, plot these point and draw a smooth curve though the point

Estimate from the graph :

(1)the midian marks

(2)the quartile marks

Find the mean of the following distribution by step deviation method:

Class Interval | 20-30 | 30-40 | 40-50 | 50-60 | 60-70 | 70-80 |

Frequency | 10 | 6 | 8 | 12 | 5 | 9 |