Hello,
The number of students who come to the school by bus is of :
1 640 * 50 / 100 = 820 students
The number of students who come to the school by car is of :
1 640 * 35 / 100 = 574 students
The total number of students who come to the school by bus or by car is of :
820 + 574 = 1 394 students
The number of pupils who go to the school on foot is of :
1 640 - 1 394 = 246 students
Bye :)