Find mean, variance from the understated data, age 30-38,39-47,48-56,57-65,66-74,75-83,84-92,no of student :10, 12, 24,30,14,6,4
Find the mean first = sum of scores/number of scores
Subtract each of the scores from the mean and square each difference. Find the sum of these squares. Divide that by the number of scores to get variance.
I'll let you do the calculations.