What is ratio of 10000 fans out of 12000 is it 5 to 1?

NO!!!!

10,000 : 12000

Cancel down the ratiOS by '1000'

Hence

10 : 12

This becomes 5:6

Hence the ratiOS are

10,000 : 12,000 :: 5 : 6

NB The colons (:) is the mathemtical symbol for 'is to'. and the double colon (::) indicates the reduced ratio.