Math Problem Statement

In the class test, marks obtained by 120 students are given in the following frequency distribution. If it is given that the mean is 59, find the missing frequencies x and y.

Solution

The question in the image asks about a frequency distribution of marks obtained by 120 students. The goal is to find the missing frequencies xx and yy, given that the mean of the marks is 59. Here's a breakdown of the problem:

Frequency Distribution Table:

Marks IntervalNo. of Students (Frequency)
0-101
10-203
20-307
30-4015
40-50xx
50-609
60-7027
70-8018
80-90yy
90-1004

Given:

  • Total number of students: 120
  • Mean marks: 59

Steps to Solve:

  1. Calculate the assumed mean and class intervals: We will use the formula for the mean of a grouped frequency distribution:

    Mean=fixifi\text{Mean} = \frac{\sum f_i x_i}{\sum f_i}

    where:

    • fif_i = frequency of the class
    • xix_i = midpoint of the class intervals

    The midpoints xix_i of the class intervals can be calculated as: Midpoint=Lower limit+Upper limit2\text{Midpoint} = \frac{\text{Lower limit} + \text{Upper limit}}{2}

    So, for each class interval, the midpoints are:

    Midpoints=[5,15,25,35,45,55,65,75,85,95]\text{Midpoints} = [5, 15, 25, 35, 45, 55, 65, 75, 85, 95]

  2. Set up the equation for the mean: We know the mean is given as 59, and the sum of frequencies (i.e., total number of students) is 120. So, the equation becomes:

    59=fixi12059 = \frac{\sum f_i x_i}{120}

    Substituting known values, we can form an equation in terms of xx and yy.

  3. Calculate the total number of students: From the frequencies, the sum of the known frequencies is:

    1+3+7+15+9+27+18+4=841 + 3 + 7 + 15 + 9 + 27 + 18 + 4 = 84

    Since the total number of students is 120, we can deduce that: x+y=12084=36x + y = 120 - 84 = 36

  4. Form the equation for the mean: Using the midpoints and frequencies, the equation for the sum of the products of midpoints and frequencies will be:

    (1×5)+(3×15)+(7×25)+(15×35)+(x×45)+(9×55)+(27×65)+(18×75)+(y×85)+(4×95)(1 \times 5) + (3 \times 15) + (7 \times 25) + (15 \times 35) + (x \times 45) + (9 \times 55) + (27 \times 65) + (18 \times 75) + (y \times 85) + (4 \times 95)

    Simplify this equation, and use the given mean (59) to solve for xx and yy.

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Math Problem Analysis

Mathematical Concepts

Statistics
Mean of grouped data
Frequency distribution

Formulas

Mean of grouped data formula: Mean = (Σf_i * x_i) / Σf_i
Midpoint of class interval: (Lower limit + Upper limit) / 2

Theorems

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Suitable Grade Level

Grades 10-12