"They cook eighteen eggs."
Translation:הם מבשלים שמונה עשרה ביצים.
Wait! The previous comments can't be right, But is Duolingo's answer right? ביצה is a feminine noun with an irregular plural ביצים. To agree with a feminine noun, the correct form of the number is the feminine עשר, not the masculine עשרה. The only way Duolingo's answer could be right is if the plural of the feminine noun ביצה is masculine both in form and for agreement. Is that so? But the way, the pop-up says that שמונה עשרה is feminine, which is surely false. Native speaker help please!
Please read the tips and instruction of this section again. It is true that by themselves the word עשרה (ASARA) means masculine 10 and the word עשר (ESER) means feminine 10, but in a construct עשרה (ESRE) means feminine 10 which will be added to the feminine preceding number and עשר (ASAR) means masculine 10 which will be added to the masculine preceding number.