my average results from the last 4 maths exams was 18 out of 25.what mark do i need in my fifth exam in order to increase the everage by 3%

Answer 1

Your average was 18/25 = 72% and you want to raise it to 75%. You need a grade x (in percent) such that

4*72 + x = 5*75
x = 5*75 - 4*72 = 87

Answer 2

An average of 18 out of 25 is 72%. I need to increase that by 3% to 75% by the time the marks from my final exam are included. Each exam is scored out of 25. Then so far I've got 72% out of a possible 100 marks, which is 72 marks. I need to get 75% overall by the time I've taken the fifth exam, which means I need to get a total of 75% x (5x25) = 93.75 marks. I've got 72 of them so far, so I need another 93.75 - 72 = 21.75 marks, which is 87%. (Note that I've assumed that every exam is scored out of the same number of marks: if they're not, then this calculation wouldn't work.)