In a class, the ratio of the number of boys to the number of girls is 3 : 4. There are 21 boys in the class. What is the ratio of the number of girls to the total number of pupils in the class, in its simplest form?
- 4 : 7
- 3 : 7
- 4 : 3
- 7 : 4
Need a hint?
Find how many pupils one part stands for first. Then compare girls with the total, not with the boys.
Worked answer
- Boys : girls = 3 : 4, and there are 21 boys, so 3 parts = 21.
- 1 part = 7, so girls = 4 parts = 28.
- Total pupils = 21 + 28 = 49, which is 7 parts.
- Girls : total = 28 : 49.
- Divide both terms by 7: 28 : 49 = 4 : 7.
Answer: 4 : 7
The mistake this question tests
Habit
Simplest form in final fraction answers
Pupils often compare girls with boys (4 : 3), or compare boys with the total (3 : 7) instead of girls.
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