Question

A number minus 20 times its reciprocal is 1. Find the number.

Answer 1

x-20/x=1

Answer 2

Undefined for all real values of x. Because when you solve the equation, you get 0 = 20 or 0 = -20, and you end up with nonsense

Answer 3

n - 20 * 1/n = 1
n - 20/n = 1
times everything by n
n^2 - 20 = n
n^2 - n - 20 = 0
(n-5)*(n+4) = 0
n1 = 5; n2 = -4

But if the question is:
(n - 20) * 1/n = 1
then
(n-20)/n = 1
times everything by n
n-20 = n
n - n = 20
0 = 20 which is nonsense O_o because 0 is not equal to 20 O_o