When a missing class is given as a percentage of all observations, the other classes make up the rest of the percentage. Add the known frequencies, match them to that remaining share, and find the total before calculating the missing frequency.
The 2027 SEC G3 Mathematics syllabus (K310) includes reading histograms with equal class intervals. In the original exercise below the vertical axis is labelled frequency, so the known bar heights give the counts directly.
- Check the vertical axis and the class widths.
- Match the known frequencies to the correct percentage of the total.
- Verify the recovered frequency against both the total and the percentage.
Worked example | A missing bar represents 25%
A frequency histogram shows four equal time intervals for a class activity. The known frequencies are 7, 11 and 12. The missing bar contains 25% of all observations. How many observations does it represent?
The three known bars total 30 observations. They represent 100% − 25% = 75% of the total. If 75% is 30, then 25% is 10 and the whole is 40.
The missing bar has frequency 10. Check that 7 + 11 + 12 + 10 = 40 and that one quarter of 40 is 10. Taking one quarter of 30 would use the known subtotal as the whole.
Find the group named by the percentage
If the wording instead said the missing class contained 25% as many observations as the other classes combined, the reference group would be different. The 30 would then be the amount to multiply by 25%. In a count question, a fractional result can also flag that you should reread the wording or check whether the percentages are rounded.
A histogram groups observations into intervals. Finding a missing bar recovers how many observations lie in that class. It leaves their individual values unknown.
Use a different percentage
The known bars total 42 observations, and the missing bar contains 30% of all observations. The known bars therefore represent 70%. The whole is 42 ÷ 0.70 = 60, so the missing frequency is 18.
Ask a study partner to explain why 42 belongs beside 70% before either of you uses the calculator. That pairing is the main decision in the question.
Frequently asked questions
Can I use the class midpoint as its frequency?
The midpoint is a value on the horizontal scale. Frequency counts observations.
What if the vertical axis says frequency density?
Read the axis first. The direct bar-height method here assumes the axis is labelled frequency.
Official information
SEAB’s 2027 SEC G3 Mathematics syllabus (K310), S1 Data Handling and Analysis. Checked 10 November 2026.