Question 477322
Marks in four Projects = 75, 90, 77, 88
The average of all five projects = 83
Let Marks in fifth project = x

Average of all five projects = sum of the marks of all five projects / total number of projects

83 = (75+90+77+88+x)/5
83 = (330+x)/5
Multiply by 5 both sides
5*83 = 5*(330+x)/5
5*83=330+x
415=330+x
415-330=x
85=x
x=85

He must score 85 marks in fifth project to get an average of 83.