Find the missing number in a decimal equation: Solve for ? in 16.7 + 12.38 − ? = 10.09, showing clean decimal subtraction and addition.

Introduction / Context:
This is a straightforward decimal arithmetic problem. It tests accuracy in adding and subtracting decimal numbers and rearranging an equation to isolate the unknown. Such items are common in aptitude sections to assess precision and speed.

Given Data / Assumptions:

• Equation: 16.7 + 12.38 − ? = 10.09
• All values are exact decimals.
• The goal is to solve for ?.

Concept / Approach:
Rearrange the equation to isolate the unknown on one side. Use the identity a + b − x = c ⇒ x = a + b − c. Be careful with decimal alignment when adding and subtracting to avoid place-value errors.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• 17.89: Off by 1.10; likely mis-subtraction.
• 16.98: Incorrect aligning of decimal places.
• 20.09: Confuses add/subtract order.

Common Pitfalls:

• Not aligning decimals precisely when subtracting.
• Dropping trailing zeros (e.g., 16.70) and miscounting place values.

Final Answer: