The problem asks to find the length of segment $RS$ given that:

• $ST = 15$
• $RT = 42$

From the number line, we can deduce that $RT = RS + ST$. Using this relationship:

$RT = RS + ST$

Substitute the values:

$42 = RS + 15$

Now, solve for $RS$:

$RS = 42 - 15 = 27$

Thus, the length of $RS$ is 27.

Would you like further details or clarification?

Here are five related questions:

1. If $RS = 27$, what would $ST$ be if $RT = 50$?
2. How would the formula change if $RT$ was smaller than $ST$?
3. Can you apply the same principle to find distances in three-dimensional space?
4. What if $RS$ was a fraction of $RT$? How would you solve it?
5. Can you think of other geometric properties of line segments that can be calculated this way?

Tip: Always check that your relationship between the segments is consistent with the visual layout of the number line.