The sum of angle in a circle = 360 degree
The total grade number of students = 45+25+60+50+60+45 = 285
The measure of the central angle for each student = (360/285) x 1 = 1.26 degree
In 7th grade, the number of student = 50, so the measure of central angle for grade 7th students = 50 x 1.26 = 63 degree