Question 88840
Let x=sum of the group who scored an average of 80 

and y=sum of the group who scored an average of 70


First lets find the sum of the 35 junior's test scores who scored an average of 80

{{{x/35=80}}} Start with the given average


{{{x=2800}}} Multiply both sides by 35


So the sum of the group who scored an average of 80 is 2,800



Now lets find the sum of the 15 junior's test scores who scored an average of 70


{{{x/15=70}}} Start with the given average


{{{x=1050}}} Multiply both sides by 15


So the sum of the group who scored an average of 70 is 1,050



Now lets average the 2 groups:


{{{(2800+1050)/50=77}}}


So the average score for all 50 is 77