dreieinhalb
three and a half
The number 3.5; refers to an amount made up of three wholes and one half.
Die Fahrt zum Flughafen dauert ungefähr dreieinhalb Stunden.
The drive to the airport takes about three and a half hours.

three and a half
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three and a half
The number 3.5; refers to an amount made up of three wholes and one half.
Die Fahrt zum Flughafen dauert ungefähr dreieinhalb Stunden.
The drive to the airport takes about three and a half hours.

Numerals like 'dreieinhalb' (three and a half) are formed according to the pattern 'whole number + ein + halb' and are always written in lowercase. The same applies to 'viereinhalb' (4.5), 'zehneinhalb' (10.5), and so on.
Der Kuchen muss für eineinhalb Stunden in den Ofen.
The cake has to go in the oven for one and a half hours.
The word 'dreieinhalb' (three and a half) is invariable, so it is not declined. The noun that follows it is always in the plural.
Wir haben noch dreieinhalb Liter Milch im Kühlschrank.
We still have three and a half liters of milk in the fridge.
'Dreieinhalb' (three and a half) is very often used with time expressions or units of measurement. Typical combinations are 'dreieinhalb Jahre' (three and a half years), 'dreieinhalb Wochen' (three and a half weeks), 'dreieinhalb Kilo' (three and a half kilos), or 'dreieinhalb Meter' (three and a half meters).
Er arbeitet seit dreieinhalb Jahren in dieser Firma.
He has been working at this company for three and a half years.