Learn Farsi: پنجاه خودکار و شصت مداد داخل کیف است. - Fifty pens and sixty pencils are inside the bag.

Fifty is پنجاه (panjāh) and sixty is شصت (shast). You can see they are built from the single-digit numbers for five (پنج, panj) and six (شش, shesh). In Farsi, the tens are often made by adding a suffix like ـاه (-āh) or ـت (-t) to the single digit, though the exact spelling and pronunciation can change a little bit.

Why doesn't the sentence use the plural forms for pens and pencils?

In Farsi, whenever you use a specific number before a noun, the noun always stays in its singular form. So instead of fifty pens, you literally say fifty pen (پنجاه خودکار) and sixty pencil (شصت مداد).

The English translation says are, but the Farsi sentence ends with ast (is). Why is the verb singular?

In Farsi, when the subject is inanimate (meaning it is not a human or an animal), the verb is almost always singular, no matter how many items there are! Because pens and pencils are objects, the sentence uses the singular verb است (ast) instead of the plural هستند (hastand).

How do the words for inside and bag connect in this sentence?

They are linked by the ezāfe, which is a short e sound. To say inside the bag, you pronounce it as dākhel-e kif. Even though this e sound is not written out as a separate letter in the Farsi alphabet, it acts as a bridge connecting the location word (داخل) to the noun (کیف).