Let a = # of singers for school A;  b = #  of singers for school B.


Then from the condition, you have these equations

       a +    b = 40

    0.4a + 0.2b = 0.325*40


or, equivalently

       a +    b = 40     (1)

    0.4a + 0.2b = 13.    (2)


From equation (1), express b = 40-a  and substitute it into equation (2), replacing "b" there


    0.4a + 0.2*(40-a) = 13

    a =  = 25.


ANSWER.  25 singers are for school A;  the rest,  40-25 = 15 are for school B.

Let number of School B-singers, be B

Then number of School A-singers is, 40 - B

We then get: .2B + .4(40 - B) = 40(.325)

.2B + 16 - .4B = 13     

.2B - .4B = 13 - 16

- .2B = - 3

Number of School B-singers, or 

You should now be able to find the number of School A-singers! 

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