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June 2011 p61 q6
2476
There are 5000 schools in a certain country. The cumulative frequency table shows the number of pupils in a school and the corresponding number of schools.
Number of pupils in a school
\(\leq 100\)
\(\leq 150\)
\(\leq 200\)
\(\leq 250\)
\(\leq 350\)
\(\leq 450\)
\(\leq 600\)
Cumulative frequency
200
800
1600
2100
4100
4700
5000
Draw a cumulative frequency graph with a scale of 2 cm to 100 pupils on the horizontal axis and a scale of 2 cm to 1000 schools on the vertical axis. Use your graph to estimate the median number of pupils in a school.
80% of the schools have more than \(n\) pupils. Estimate the value of \(n\) correct to the nearest ten.
Find how many schools have between 201 and 250 (inclusive) pupils.
Calculate an estimate of the mean number of pupils per school.
Solution
(i) To find the median, draw the cumulative frequency graph using the upper class boundaries and cumulative frequencies. The median is the value at the 2500th school (since there are 5000 schools). From the graph, the median number of pupils is approximately 270.
(ii) 80% of 5000 schools is 4000 schools. From the cumulative frequency table, 4000 schools correspond to a number of pupils less than 160. Therefore, \(n = 160\).
(iii) The number of schools with pupils between 201 and 250 is given by the difference in cumulative frequencies: \(2100 - 1600 = 500\).
(iv) To estimate the mean, use the midpoints of the intervals: \((50.5 \times 200 + 125.5 \times 600 + 175.5 \times 800 + 225.5 \times 500 + 300.5 \times 2000 + 400.5 \times 600 + 525.5 \times 300) / 5000 = 268\).