Quartiles from frequency tables

The ages of 80 members of a sports club are in the classes 20–, 30–, 40–, 50–, 60–70 with frequencies 9, 21, 28, 16, 6. Build the cumulative column and find Q₁, Q₂ and Q₃.

Heights of 40 seedlings in cm: classes 10–, 20–, 30–, 40–, 50–60 with frequencies 5, 9, 13, 8, 5. Find the three quartiles with the formula.

The curve shows the daily steps, in hundreds, of 60 walkers. Read Q₁, Q₂ and Q₃ (orders 15, 30 and 45). The table behind it has the classes 0–, 20–, 40–, 60–, 80–100 with frequencies 6, 14, 20, 12, 8: check one of your readings with the formula.

A pupil has 50 values in a frequency table and finds the orders (50 + 1) ÷ 4 = 12.75, 25.5 and 38.25. What should she use instead, and what are the orders?

A table has the classes 0–, 10–, 20–30 with frequencies 10, 20 and 10. Find the three quartiles. Notice what happens with the order 10.