it was said that 78% was the boy's mean.
therefore, sum of the five tests over 5 is 78%.
but since the fifth one was erased, let it be equal to x.
so, sum of the four exams plus x all over 5 is still 78%.
hence, sum of the four exams plus x = 5* 78% = 390%.
therefore, x= 390% - sum of the four exams.. Ü