The question is: If the student is known to be the girl (300 girls) what is the probability of being right handed.
We know that there are 300 girls, and that out of those 300, there are 250 girls right handed.
Divide 250 with 300
250/300 = 0.83333, or 83.33%
0.83333, or 83.33% is your answer
hope this helps