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x = (x-20)^2
Answer. x = 25 (obtained mentally).
Algebra solution
x = x^2 - 40x + 400
x^2 -41x + 400 = 0
(x-25)*(x-16) = 0
Of two roots of this quadratic equation x= 25 and x= 16 only x= 25 remains positive "20 years ago".