Hemi-, demi- and semi- are prefixes which mean <u>half</u>.
When applied to musical notation, each time a prefix is added it means the note's value is <u>halved</u>.
Therefore, if a quaver is one-eighth of a semibreve, then a <u>semiquaver</u> is half of a quaver, which means it's <u>one-sixteenth of a semibreve</u>.
Therefore, a hemisemidemiquaver is one-sixty-fourth of a semibreve.
Basically when comparing a decimal from least to greatest look at the tenths place. Whoever's is the smallest, is the smallest decimal. Then you look at the second smallest etc. If two tenths are the same then you would compare the hundredths place so on.