Question 868422
Take note of the formula to calculate average as follow.
(Total number of Score)/(Total number of students) = Average Score

For boys,
(Total Number of Boy Score)/60 = 55
Total Number of Boy Score = 55 X 60
Total Number of Boy Score = 3300

For Girls,
(Total Number of Girl Score)/40 = 45
Total Number of Girl Score = 45 X 40
Total Number of Girl Score = 1800

Total number of Score for both Boys and Girls = 3300 + 1800 
Total number of Score for both Boys and Girls = 5100
Knowing that The total number of boys and girls students = 60 + 40 = 100 students
Thus, 
Overall average score = (Total number of Boys and Girls Score)/(Total Number of Boys and Girls)
Overall average score = 5100/100
Overall average score = 51