The cumulative frequency curve shows the marks obtained by the 120 students in a recent Mathematics Test. The maximum mark is 100.

1. (a)

   Complete the grouped frequency table for the marks obtained.

   Marks obtained by students, x	Frequency
   0 ≤ x < 20	10
   20 ≤ x < 40
   40 ≤ x < 60
   60 ≤ x < 80	28
   80 ≤ x < 100	8

2. (b)

   Calculate an estimate of the mean mark.

   0 ≤ x < 20

   20 ≤ x < 40

   40 ≤ x < 60

   60 ≤ x < 80

   80 ≤ x < 100

   Answer ... marks

3. (c)

   Calculate an estimate of the standard deviation.

4. (d)

   The passing mark is 50. Two students are chosen at random. Find the probability that both of them passed.