Thank you for your answers everyone. I do understand the "half hour" and "half an hour" etc sayings - my main concerns we around the use of "a hundred dollars" or "one hundred dollars" or other examples similar, (there were "a hundred" people present...... or .... there were "one hundred" people present....) But it does seem widely accepted that "a" or "one" can both be used and be correct. Cheers.