How to write a numerical or variable expression for each quanity? For example two dozen eggs and how do you know if your writing a num. or a var....

A numerical value is any number.  It is known. A numerical
value is fixed. A numerical value is a constant. A numerical value could be final
solution for a variable.

Example :  A number like  five,
ten , half,  a quarter etc. Any special numerical expression like 2+2, 5+10^2 ,  a dozen
, a crore,  etc.

A variabe is any quantity assumed. We
assume that x is the number that has to be added t0 5 to get thirteen. So we solve x+ 5
= 13 . So we can assume any value for the unknown x of  the equation , it is a variable.
But x = 8 is the only one value for which x+8 = 13 holds true. So now the solution x = 8
is a constant. The set of all values that could be  assumed   form  the variable  x set
. But a particular value ( here the solution of x, i.e , 8)  is a solution to x+8 = 13
which is a constant .