"I would like a kilogram and a half of apples."
Translation:Je voudrais un kilo et demi de pommes.
I think I wrote this lesson zillions of times already, so I am copying and pasting what follows from another thread: https://forum.duolingo.com/comment/31998869
"Il y a des biscuits" is the plural of "il y a un biscuit". Therefore, the "des" is the plural indefinite article English does not have.
When the verb is in the negative, "des" becomes "de":
- Il n'y a pas/plus/jamais de biscuits.
When there is an adjective before the noun, "des" becomes "de":
- Il y a de bons biscuits.
When there is an expression of quantity including "de" before the noun, "des" disappears:
- Il y a un peu de/autant de/moins de/assez de/plus de/peu de/beaucoup de... biscuits.
When a verb phrase includes "de", "des" disappears:
- J'ai besoin de biscuits
- J'ai envie de biscuits
With the "noun of noun" construction, where the second noun gives further information on the first noun in terms of its content, quality, material or purpose, "de" replaces "des":
- Une boîte de biscuits
- Une recette de biscuits
So "un kilo et demi de pommes" belongs to the latter rule.