Question

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

Answer 1

When you're solving for an unknown variable, it's generally seen to use x first, then y is there is already an x, followed by z, and then moving to a and going down the alphabet from there depending on how many variables are necessary. However, any letter or symbol would technically work as the placeholder for the unknown.

Answer 2

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

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.