Break the shape into two recognizable shapes. In this case a triangle and a square. To find area of a triangle use Area= 1/2 base x height. To find area of a square use Area = length x width. Then add the two together.
30% of 50 is 15 beacuase 50 is 1/2 of 100, and of 100 students 30 would have submtted forms. 1/2 of 30 is 15 which s the number of students that turned n forms