The number of customers in one morning at 60 tea stalls in Kolkata is: 0-20: 5, 20-40: 12, 40-60: 20, 60-80: 15, 80-100: 8. Find the mode.

1. 1 · Write what is given and what is to be found. Given: five classes of size 20; 60 stalls in all. To find: the mode.
2. 2 · Find the modal class. The highest frequency is 20, so the modal class is 40-60.
3. 3 · Write the formula. Mode = l + ((f₁ − f₀) ÷ (2f₁ − f₀ − f₂)) × h.
4. 4 · Write the values. l = 40 (lower limit of the modal class), h = 20, f₁ = 20, f₀ = 12, f₂ = 15.
5. 5 · Substitute. Mode = 40 + ((20 − 12) ÷ (40 − 12 − 15)) × 20 = 40 + (8 ÷ 13) × 20 = 40 + 12.31 = 52.31.
6. 6 · Say what it means. The most common number of customers at a stall is about 52.
7. 7 · Check and box the answer. 52.31 lies inside 40-60. Box: Mode ≈ 52.31.

Scoring tip: Write l, h, f₁, f₀ and f₂ on one line before you substitute. Using a wrong neighbour frequency is the most common error.