The value of the unknown number is 10.5.
Let the unknown number = x
When 20 is subtracted from the number and the result is squared which is equal to the square of the unknown number minus 20:
(x - 20)² = x² - 20
expand the equation;
x² - 40x + 400 = x² - 20
collect similar terms together;
x² - x² - 40x = -20 - 400
-40x = -420
[tex]x = \frac{420}{40}\\\\ x = 10.5[/tex]
Thus, the value of the unknown number is 10.5.
Learn more about word problem here: https://brainly.com/question/18336356
