239 - a real expert

A giant ship's engine broke down and no one could repair it, so they took it to a Mechanical Engineer with over 40 years of experience. He inspected the engine very carefully, from top to bottom. After seeing everything, the engineer unloaded the bag and pulled out a small hammer. He knocked something gently. Soon, the engine came to life again. The engine has been fixed! 7 days later the engineer mentioned that the total cost of repairing the giant ship was $ 10,000 to the ship owner "What ?!" said the owner. "You did almost nothing. Give us a detailed bill." The answer is simple: Tap with a hammer: $ 2 Know where to knock & how much to knock: $ 9,998