If there were one class of numbers responsible for students giving up on the mathematics discipline entirely, it would be the imponderables.
Perplexing numbers are the most benign form of imponderable. They are numbers that are answers to questions you should know the answer to, but don’t, for example why is the sky blue or is tinsel bio-degradeable?
The largest class of imponderables is the vexing numbers. Vexing numbers are different from perplexing ones in that they appear in questions that have to be answered, like “To what do the numbers on this court order refer?” Or all exam questions.
Although some impenetrable numbers are vexing, they are usually considered as a separate class because most refer to obscure things, like port settings on your younger brother’s web server. A debate rages as to whether impenetrability is a characteristic of the observer (aka the dense mind hypothesis) or the number itself.