Half the pointer, a changing source current

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Problem: Half the pointer, a changing source current
A linear galvanometer is in series with R = 600 Ω and a stable ideal source. A shunt S = 120 Ω is connected across its coil; the pointer moves from 24 to 12 divisions without changing R.
Question: What is the coil resistance G?
Givens:
- R = 600 Ω
- S = 120 Ω
- Deflection ratio = 1/2
Assumptions: Ohmic fixed-temperature resistances; negligible source/lead resistance; same emf and series setting in both states.
Choices (LaTeX):
A. 120\ \Omega
B. 100\ \Omega
C. 150\ \Omega
D. 480\ \Omega

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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.