In a description of a fluid circulation system, a sentence below is encountered:<br/><br/>... consists of flow lines,...... the single mass flow is divided into <b>first and second</b> flow lines,...<br/><br/>Question: why is there not a "a" before "first" and "second"? If there is, any difference in meaning? Which is correct?<br/><br/>the single mass flow is divided into <b>a first and a second</b> flow lines??