What letter by convention is used for a varible when another named isn't obvious

By convention, x is used for a variable when another named isn't obvious.

Step-by-step explanation:

Generally the alphabet x is used for a variable when another named isn't obvious.

The X was used for unknown variables due to inability of scholars of Spain to translate some arabic words.

In Arabic unknown thing is termed as al-shalan. As Spanish scholars did not had any sound for 'sh', they pronounced it as 'ck' sound, which is denoted as X (chi symbol).  

For some cases the variable defined according to the problem. For example

1. For speed problem, we use variable s.

2. For time problem, we use variable t.

3. For proportion problem, we use variable p, etc.

Therefore, by convention x is used for a variable when another named isn't obvious.

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