Generally speaking, there are no unknowns with fixed terms (such as quantity, number of people, total cargo, etc.). ) usually set in the order of x, y, z, (u, v,) w.

For an unknown with a fixed term, it is usually set according to the initial letter of the term, such as the radius of the circle is set to r, and the mass of the object is set to m, etc. Of course, it can also be set to x, y or something.

Unknown integers are usually set to k, (l,) m, n; Rational numbers are usually set to p, q, r, s, t, etc. It can also be used as the setting of unknowns, which will play an important role, such as "find P when taking congruence." .

The letters in brackets above are usually used when other letters are not enough, for example, when only four unknowns are needed, xyzw is used; Use xyzuv for five hours.

As for other letters, abcd is usually used as the known number, e-j is not used much, and O is not used, because it is easy to be confused.