To find the mean, you add up the numbers and divide by
how many numbers there are.
So if the mean of 'A' and 'B' is 40, then A + B = 80 .
And if the mean of 'B' and 'C' is 35 then B + C = 70.
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You said that (A + B) + C = 100 .
From something else that you said, I noticed that A + B = 80 .
So I can write (80) + C = 100
Subtract 80 from each side: C = 20
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I also noticed, from what you said, that B + C = 70 .
So I can write B + 20 = 70
Subtract 20 from each side: B = 50
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Now I know that C = 20 and B = 50.
Finally, you said that A + B + C = 100 ,
So I can write A + 50 + 20 = 100
Combine like terms on the left: A + 70 = 100
Subtract 70 from each side: A = 30
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A = 30
B = 50
C = 20