Explanation:
a full circle has 360°.
so, since each individual result is represented by an amount of degrees, the total sum of all results must be 360°.
labour = 360 - 136 - 34 - 54 - 14 = 122°
the fraction 122/360 represents the 183 votes for labour.
122/360 = 183
the fraction 1/1 = 1 then represents the total number of votes in the whole system.
61/180 = 183
1/180 = 3
1 = 3×180 = 540
540 pupils voted altogether.