A, b and c stand for three different numbers
the mean of a and b is 40
the mean of b and c is 35
a +b+c=100
calculate the values of a, b and c

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.

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

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

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


             A = 30
             B = 50
             C = 20