Keep the last justified decimal

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Problem: Keep the last justified decimal
Two measured lengths are 2.40 m and 1.237 m. Their sum is reported using the usual decimal-place rule for addition.
Question: Which reported sum is appropriate?
Givens:
- First length: 2.40 m
- Second length: 1.237 m
Assumptions: Treat both entries as measurements with the displayed precision, not exact defined lengths.
Choices (LaTeX):
A. 3.6\ \mathrm m
B. 3.64\ \mathrm m
C. 3.637\ \mathrm m
D. 3.63\ \mathrm m

Ask for my approach first. Help me distinguish a physical misconception from an algebra error without revealing which choice is correct.

Tutoring approach:
Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.