Question 164019
If A, B, and C are the number of A model, B model, and C model desks respectively, then we know that in Jan, A + B + C = 85.  We are told that the number of b desks was five more than the number of C desks, so B = C + 5.  Furthermore, the number of A desks was four more than twice the number of C desks, so A = 4 + 2*C.  Now we just substitute for A and B in our initial equation:
{{{(4 + 2*C) + (C+5) + C = 85}}}.  This reduces to {{{9+4c = 85}}} or {{{4c = 76}}} which implies c = 76/4 = 19.  Therefore 19 c desks were sold.  We know can determine that the number of B desks sold was B = C + 5 = 19 + 5 = 24, and the number of A desks sold was A = 4 + 2*C = 4+(2*19) = 4+38 = 42.  To check, 24 + 42 + 19 = 85.
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In summary, 42 A desks were sold, 24 B desks were sold, and 19 c desks were sold.