| Marks | 11-20 | 21-30 | 31-40 | 41-50 | 51-60 | 61-70 | 71-80 |
| No. of students | 2 | 6 | 10 | 12 | 9 | 7 | 4 |
Solution:
Class mark, xi = (upper class limit + lower class limit)/2
Assumed mean, A = 45.5
| Marks | No. of students (fi) | Class mark (xi) | di = xi – A | fidi |
| 11-20 | 2 | 15.5 | -30 | -60 |
| 21-30 | 6 | 25.5 | -20 | -120 |
| 31-40 | 10 | 35.5 | -10 | -100 |
| 41-50 | 12 | 45.5 | 0 | 0 |
| 51-60 | 9 | 55.5 | 10 | 90 |
| 61-70 | 7 | 65.5 | 20 | 140 |
| 71-80 | 4 | 75.5 | 30 | 120 |
| Total | ∑fi = 50 | ∑fidi = 70 |
By short cut method, Mean = x̄ = A+∑fidi /∑fi
= 45.5+70/50
= 45.5+1.4
= 46.9
Hence the mean of the distribution is Rs.46.9.