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1. haven't done stats in ages so need some help here

Classes of students, from three different schools, sat the same examination paper. The mean scores for each school were 63, 38 and 72 %. If the respective class sizes were 23, 30 and 15, find the mean mark of the schools taken together.

How do I do this?
2. (Original post by not_lucas1)
haven't done stats in ages so need some help here

Classes of students, from three different schools, sat the same examination paper. The mean scores for each school were 63, 38 and 72 %. If the respective class sizes were 23, 30 and 15, find the mean mark of the schools taken together.

How do I do this?
Think about it this way, if there were 30 pupils each from school A and B, and if the average marks were 60 and 80, then the average between the two schools would be 70. This is because they're equal so it almost seems like you're not taking a weighted average.

Whereas if there were 15 in a class that got an average of 85, and a class of 45 that got an average of 65, then the average would be 70 (weighted towards the class with more students). So if you've not come across it before, it's worth looking up what a 'weighted average' is.

The other way to think about it is to deconstruct and reconstruct the mean. Eg if there were 15 that got 85 then the total score is 15*85 there, the other class has a total score of 45*65.. so the new total score is ...and the new total population is.. and so the new average is...

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