The divisions for the number of students are not labelled, so assume every division is 1 unit.


①This question is on average so we can use the 'donation' concept/idea

Number of students who scored 26 marks = 2 units

Difference between average score and 26 = 29 - 26
= 3

To reach the average of 29, number of marks needed

= 2 units × 3
= 6 units



②Number of students who scored 27 marks = 3 units

Difference between average score and 27 = 29 - 27
= 2

To reach the average of 29, number of marks needed

= 3 units × 2
= 6 units


③Number of students who scored 28 marks = 5 units

Difference between average score and 28 = 29 - 28
= 1

To reach the average of 29, number of marks needed

= 5 units × 1
= 5 units


④ Total marks needed to reach the average
= 5 units + 6 units + 6 units
= 17 units

So, we need a 'donation' of 17 units from the students scoring full 30 marks.


To reach the average, each of these students can 'donate' 1 mark.

(30 - 29 = 1)

Number of full-mark students needed

= 17 units ÷ 1 mark donated per student

= 17 units