# You need to be able to calculate in your head

Imagine I'm sitting in a room with some fellow engineers, going over some recent test results on a new product. (This happens a couple of times a month at work BTW).

Conversation 1: (If you can't solve basic linear equations in your head:)

Eng1: "Data point X and Data point Y are in contradiction, there must be a measurement error"

Eng2: (spends 20 seconds on a calculator or wolfram alpha distractedly typing stuff in, or is he reading his email and ignoring everyone?)

Eng3: "is he right? I'm not sure. We should answer this at the next meeting"

Eng2: (after 20 seconds of tapping) - "yes he is. Now what?"

Conversation 2: you can solve basic linear equations in your head

Eng1: "Data point X and Data point Y are in contradiction, there must be a measurement error"

Eng2: 3 seconds later "yes, you are correct"

Eng3: "I know what must be wrong with the measurement. If we interpolate..."

See the difference? The human-computer interaction is *slow*. The interaction in your head is substantially faster. You can't carry on conversations with others if they are constantly referring to an electronic device.

For those of you who gamble to win, the same applies to poker. You have so many seconds to decide. Even if they allowed the use of a calculator or computer, it would be too slow, you have to do it in your head.

Maybe this doesn't have to involve using paper. But learning all the basic arithmetic tables is prerequisite, as well as the other basic arithmetic calculations.

This is the reason that Feynman forced himself to be able to do basic calculations in his head. Everything goes faster allowing you to iterate to a solution quickly. (He originally did it for gambling BTW!)