Gifted Engineer

There was an engineer who had an exceptional gift for fixing machines.
After serving his company loyally for over 30 years, he happily retired.
But several years later they contacted him regarding a seemingly impossible problem they were having with one of their multimillion dollar machines. They had tried everything, but to no avail.
The engineer reluctantly took on the challenge. He spent a day studying the huge machine.
At the end of the day he marked a small X in chalk on a particular component of the machine and proudly stated, "This is where your problem is!"
The part was replaced and the machine worked perfectly again.
The company received a bill for $50,000 from the engineer for his services.
They demanded an itemized accounting of his charges.
The engineer responded briefly: "One chalk mark: $1. Knowing where to put it: $49,999."