When do the two carts meet?

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Problem: When do the two carts meet?
Two carts 150 m apart approach each other on one straight track at 6 m/s and 4 m/s.
Question: Find the meeting time.
Givens:
- initial gap = 150 m
- speeds = 6 and 4 m/s
Assumptions: Both carts move at constant speeds toward each other on the same straight line.
Choices (LaTeX):
A. 25\ \mathrm s
B. 37.5\ \mathrm s
C. 15\ \mathrm s
D. 75\ \mathrm s

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.