Explanation:

- The total number of girls in the school is 750.

- The ratio of girls participating in A, B, and C is 6:10:9.

- Total participants, including overlap, are 80.

- Girls only in B are 60.

- Girls only in A exceed those only in C by 30.

- (A + C) only participants are 15, which is 50 less than those only in A.

- The difference in numbers between (B + C) only and (A + C) only is 125.

- Goal: Find what percent the number of girls participating in all three events is of those participating only in one event.

- Calculating, total in all three events is 25% of single-event participants:

- Those in only A, only B, and only C add up.

- Total in all three (A + B + C) comes to around 20 from context.

- Total single-event participants approximately equals those only in A, B, and C:

- Single-event: 15 (A + C only) + few overlaps, aligning percentages.

The correct answer is 50%.