Add the 3 stages for the total number of students:
380 + 240 + 110 = 730 total students.
The fraction of students in Key Stage 3 = 380 / 730
She needs a sample of 100, so multiply the fraction of Stage 3 by the sample rate:
380/730 x 100 = 52.05 people.
Since you cannot have 0.05 of a person, depending on how your teacher wants you to round you could have either 52 or 53 people.