"syntax" refers to the rules on how words are put together to form a coherent sentence. In regard to numbers, there are several different ways that could be interpreted. First there is the whole point of how to form numbers using numerals in base 10. That is a "syntax" question. Second, there are rules governing how mathematical expressions are put together. For example (3+ ((x- )) will give "syntax" errors in a computer program! Finally, there are rules on how numbers are written or spoken. The number 342.32 should be written out "three hundred forty two and thirtytwo hundredths", NOT "three hundred and thirtytwo and point three two. That is a matter of "syntax".