The ratio 3:2 means that out of every 5 students, 3 are boys and 2 are girls. Since the total number of students is 30, we first find how many groups of 5 there are: 30 ÷ 5 = 6. The number of boys is then 3 × 6 = 18. Hence, there are 18 boys in the class.
Option A:
If there were 12 boys, then the ratio of boys to girls would not be 3:2 with a total of 30 students. Specifically, 12 boys would leave 18 girls, giving a ratio of 12:18, which simplifies to 2:3, not 3:2. Therefore, this option does not satisfy the given ratio.
Option B:
If there were 15 boys, the remaining 15 students would be girls, resulting in a ratio of 15:15 or 1:1. This is very different from the required ratio of 3:2. Thus, option B is not correct.
Option C:
18 is correct because it maintains the ratio 3:2 with a total of 30 students. With 18 boys and 12 girls, the ratio 18:12 simplifies to 3:2 as required. This method demonstrates how ratio and total together determine individual quantities in aptitude problems.
Option D:
If there were 20 boys, only 10 girls would remain, giving a ratio of 20:10 or 2:1, which does not match 3:2. This inconsistency shows that 20 is not the correct number of boys.
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