The phrase "twice a number less than 8 is 21" can be translated into a mathematical equation. Let's break it down:

• "Twice a number" means $2x$ (where $x$ is the unknown number).
• "Less than 8" suggests that we subtract $x$ from 8, resulting in the expression $8 - x$.
• The entire expression is set equal to 21.

The equation is:

$2(8 - x) = 21$

Now, let's solve the equation:

$2(8 - x) = 21$ $16 - 2x = 21$ $-2x = 21 - 16$ $-2x = 5$ $x = -\frac{5}{2} \quad \text{or} \quad x = -2.5$

So, the number is $-2.5$.

Would you like more details or have any questions?

Here are some related questions:

1. What if the number is increased instead of decreased by 8?
2. How would the equation change if "twice the number" was equal to 16 instead of 21?
3. Can you solve the equation if it was "three times a number less than 10 is 21"?
4. How would you graph the equation $2(8 - x) = 21$?
5. What would the equation be if the phrase was "twice a number greater than 8 is 21"?
6. How does the solution change if the phrase was "half a number less than 8 is 21"?
7. What is the significance of the negative solution in this context?
8. Can you verify the solution by plugging it back into the original equation?

Tip: When solving equations involving negative numbers, carefully handle the signs to ensure accuracy.