The required number is 7. The expression formed with the given information is x - 21 = 2(-x).
An expression is the combination of variables, constants, and coefficients.
If two expressions are related by an equal in between them, then that is said to be an equation.
From the given data,
Consider the required number = x
The number is decreased by 21 i.e., x - 21
Twice the opposite of the number = 2(-x)
So, on equating,
x - 21 = 2(-x)
On simplifying,
⇒ x - 21 = -2x
⇒ x + 2x = 21
⇒ 3x = 21
∴ x = 7
Thus, the required number is 7.
Check:
7 - 21 = -14 and 2(-7) = -14.
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