Shortcut Trick
Using the Assumed Mean Method, let the assumed mean (A) = 35 and class width (h) = 10.
The deviations (ui) for the midpoints (15, 25, 35, 45, 55) are −2, −1, 0, 1, 2 respectively.
Σfiui = (8 × −2) + (12 × −1) + (16 × 0) + (11 × 1) + (13 × 2) = −16 − 12 + 0 + 11 + 26 = 9.
Mean = A + (Σfiui ÷ Σfi) × h = 35 + (9 ÷ 60) × 10 = 35 + 1.5 = 36.5.
∴ The correct answer is 36.5.
Alternate Method
Given:
The frequency distribution of the given class intervals.
Formula Used:
Mean = Σ(fi × xi) ÷ Σfi
where xi is the midpoint of each class interval and fi is the corresponding frequency.
Calculations:
Find the midpoints (xi) of each class interval:
⇒ For 10 - 20: x1 = (10 + 20) ÷ 2 = 15
⇒ For 20 - 30: x2 = (20 + 30) ÷ 2 = 25
⇒ For 30 - 40: x3 = (30 + 40) ÷ 2 = 35
⇒ For 40 - 50: x4 = (40 + 50) ÷ 2 = 45
⇒ For 50 - 60: x5 = (50 + 60) ÷ 2 = 55
Calculate the product of frequency and midpoint (fi × xi):
⇒ f1 × x1 = 8 × 15 = 120
⇒ f2 × x2 = 12 × 25 = 300
⇒ f3 × x3 = 16 × 35 = 560
⇒ f4 × x4 = 11 × 45 = 495
⇒ f5 × x5 = 13 × 55 = 715
Calculate the sum of frequencies (Σfi) and sum of products (Σfixi):
⇒ Σfi = 8 + 12 + 16 + 11 + 13 = 60
⇒ Σfixi = 120 + 300 + 560 + 495 + 715 = 2190
Calculate the mean:
⇒ Mean = Σfixi ÷ Σfi = 2190 ÷ 60 = 36.5
Why other options are incorrect:
⇒ Option (1) 12 is incorrect because it lies at the extreme lower end of the first interval and does not represent the central tendency of the dataset.
⇒ Option (2) 23.4 is incorrect because the higher frequencies are concentrated in the 30-60 range, pulling the mean significantly higher than 23.4.
⇒ Option (4) 42.7 is incorrect because it is higher than the actual weighted average of 36.5.
∴ The correct answer is 36.5.
Additional Information
Direct Method for Grouped Mean
The standard formula is Mean = Σfixi ÷ Σfi, where xi is the class mark (midpoint) of the i-th class interval.
Assumed Mean Method
Used to simplify calculations: Mean = A + Σfidi ÷ Σfi, where A is the assumed mean and di = xi − A is the deviation.
Step-Deviation Method
Further simplifies calculations when class widths are equal: Mean = A + h × (Σfiui ÷ Σfi), where ui = (xi − A) ÷ h and h is the class width.