Based on the calculations, the unknown number is equal to [tex]\frac{-45}{7}[/tex]
- Let the unknown number be x.
How to find an unknown number:
Translate the word problem into an algebraic expression, we have;
Adding 21 to the unknown number:
[tex]21 +x[/tex]
Multiplying the result by 3:
[tex]3\times (21+x)[/tex]
84 more than two-thirds of the unknown number:
[tex]\frac{2x}{3} +84[/tex]
Equating the equations, we have:
[tex]3\times (21+x) = \frac{2x}{3} +84\\\\43+3x=\frac{2x}{3} +84[/tex]
Cross-multiplying, we have:
[tex]43+3x=\frac{2x}{3} +84\\\\129+9x=2x+84\\\\9x-2x=84-129\\\\7x=-45\\\\x=\frac{-45}{7}[/tex]
Read more on word problems here: https://brainly.com/question/13170908
