I was given a cryptarithmetic problem by my teacher: $$\mathrm{TWO}+\mathrm{TWO}=\mathrm{FOUR}$$
I solved it as F=0, O=2, U=6, R=4, T=1 and W=3. $$\begin{array}{ccccccc} &&&&1&3&2\\ +&&&&1&3&2\\ \hline &&&0&2&6&4\\ \end{array}$$
But I was told that F can't be 0 because it is the first letter of the word.
Can anyone explain this to me? Why is there such a rule?
Because to be mathematically valid, a leading zero would be removed. $0123$ is actually $123$ in mathematics, for example.