Which number is least 145. 155 or 144.809

145.555 is the least

Do you mean 145.155 as the first number and 144.809 as the second? If so ... the least would be 144.809.

Ignore the decimal ... 145 is bigger than 144. The decimal would come into play only when the whole number is equal. For example:

144.234 or 144.334 - - which one is least? Now you'd look at the decimals to determine 144.234 is less than 144.334.