Given that angles $M$ and $R$ are complementary (sum to 90°), we can set up an equation:

1. Sum of Angles: $M + R = 90^\circ$

2. Substitute the Expressions for Each Angle: $(5x + 10) + 55 = 90$

3. Combine Terms: $5x + 65 = 90$

4. Isolate $x$: $5x = 90 - 65$ $5x = 25$

5. Solve for $x$: $x = \frac{25}{5} = 5$

So, the correct answer is: B. 5

Would you like further explanation or have questions about complementary angles?

Here are some related questions you might consider:

1. How would the equation change if angle $R$ was given as a different measure?
2. What would happen if the angles were supplementary (sum to 180°) instead?
3. How do we know if two angles are complementary based on their measures?
4. How would you solve if angle $M$ had a different expression, like $3x + 20$?
5. What steps can you take to verify your answer in complementary angle problems?

Tip: In complementary angle problems, always remember that their sum is exactly 90°.