Read a wave from its equation

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Problem: Read a wave from its equation
A wave is y = 0.020 sin(4x − 20t), with all lengths in metres and time in seconds.
Question: Find its propagation speed.
Givens:
- k = 4 rad/m
- ω = 20 rad/s
Assumptions: Travelling sinusoidal string wave; x in metres, t in seconds and phase in radians.
Choices (LaTeX):
A. 5\ \mathrm{m\,s^{-1}}
B. 80\ \mathrm{m\,s^{-1}}
C. 0.40\ \mathrm{m\,s^{-1}}
D. 0.20\ \mathrm{m\,s^{-1}}

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.