plural and singular

[Retriever]

When should plural and singular forms be used respectively? I find the usage confusing. For example,

..... das Thermometer manchmal bis zu 60 Grad (singular) (why not plural?) minus.
...... 30 Millionen (plural) Tonnen (plural) Abfälle auf den Müll.
....... sieben Millionen (plural) Liter (plural?) Bier getrunken.
....... fast 80 Kilo (singular) (why not plural?) Müll (plural?) pro Person ......


Are there any rules for the usage? Thank you for your help.