Final answer:
The travel tip that is NOT related to mobile computing is buying a used laptop computer at a garage sale.
Step-by-step explanation:
The travel tips for mobile computing listed in the options are:
- Use hands-free to decrease the radiation to the head.
- Keep the mobile phone away from the body.
- Do not use a cellphone in a car without an external antenna.
Out of the given options, the one that is NOT a travel tip for mobile computing is buying a used laptop computer at a garage sale (option c) as it does not specifically pertain to mobile computing while traveling.