Question 1140905
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            This problem is to be solved MENTALLY.



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From the condition, an attentive reader can notice momentarily, that the number of friends is 31-9 = 22.


Then the number of sheets is 4*22 + 9 = 88+9 = 97.      <U>ANSWER</U>


<U>CHECK</U>.  22*3+31 = 66 + 31 = 97, the same number.   ! Correct !
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Solved.


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Another, formal algebra solution is to write this system of 2 equations


    X - 3*N = 31,     (1)
    X - 4*N =  9,     (2)


where X is the number of sheet and N is the number of friends, and solve it by any way.
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