In a class of 90 students, the numbers of boys are twice the number of girls.…

Concept: For a ratio, the total is split into parts, and each part's actual count is (ratio share) times (total divided by sum of ratio parts). When people stand in a line and one person is used as a reference point, everyone else falls strictly to their left or strictly to their right, and the reference person is excluded from both sides — so a category's count on one side equals (total in that category) minus (count of that category on the other side) minus (1, if the reference person belongs to that category).

Application:

1. Boys : Girls = 2 : 1, so total parts = 3, giving each part = 90 ÷ 3 = 30. So Girls = 30 and Boys = 60.

2. Swathi is 58th from the left in a line of 90 students, so the number of people to her right = 90 − 58 = 32.

3. Of those 32 people to her right, 20 are boys, so the number of girls to her right = 32 − 20 = 12.

4. Swathi herself is one of the 30 girls, and she is neither to her own left nor to her own right.

5. Girls to the left of Swathi = Total girls − girls to the right − Swathi herself = 30 − 12 − 1 = 17.

Cross-check: There are 57 people to Swathi's left (since she is 58th). So boys to her left = 57 − 17 = 40. Adding both sides, boys to her left + boys to her right = 40 + 20 = 60, which matches the computed total of 60 boys, confirming the count of 17 is consistent.

Result: 17 girls are to the left of Swathi.