you know that mean is equal to the sum of the five tests divided by 5..

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