Learn Finnish: Lääke apteekissa maksaa neljäkymmentäyksi euroa. - The medicine in the pharmacy costs 41 euros.

Adding the ending -ssa (meaning "in") triggers a common spelling rule called consonant gradation. Because of this rule, the strong double kk in the base word apteekki softens to a single k when it becomes apteekissa.

How do you build a long number word like neljäkymmentäyksi?

Finnish numbers are written together as one long word. It breaks down into neljä (four), kymmentä (tens, in the partitive case), and yksi (one). It literally translates to "four tens one".

Why is the word for euro written as euroa here?

In Finnish, any number greater than one requires the noun that follows it to be in the partitive case. Because the amount is 41, the basic word euro takes the partitive ending -a to become euroa.

I thought the verb maksaa meant "to pay". Why does it mean "costs" here?

The verb maksaa actually means both "to pay" and "to cost" depending on the context. If the subject is a person, it usually means "to pay". Since the subject here is a thing (lääke, meaning medicine), it means "costs".