Learn Russian: У нас нет пяти калькуляторов и десяти маркеров. - We do not have five calculators and ten markers.

The phrase у нас нет (we don't have) always requires the genitive case for the things you are missing. Therefore, the numbers пять and десять must be put into the genitive case, becoming пяти and десяти.

How do I know which ending to use for numbers like пяти and десяти?

Numbers from 5 to 20 decline exactly like feminine nouns that end in a soft sign (like тетрадь or дверь). For these words, the genitive, dative, and prepositional cases all drop the soft sign and take the ending -и.

Why are калькуляторов and маркеров in the genitive plural? Is it just because they follow the numbers 5 and 10?

Not exactly! While numbers 5 and up do trigger the genitive plural when they are the subject or direct object of a sentence, here the entire phrase is pushed into the genitive case by нет. When a number is in an oblique case (any case besides nominative or inanimate accusative), the noun must match the number's case. So here, we simply have a genitive number paired with a genitive plural noun.

Does the pronunciation of these numbers change in the genitive case?