Learn Turkish: Şirketin geliri onda bir azaldı. - The company's income decreased by one-tenth.

In Turkish, fractions are formed using the locative case, which is the suffix used for "in" or "at". Onda bir literally translates to "in ten, one". To build any fraction, you always say the bottom number first with the locative ending, followed by the top number.

Why does the sentence say şirketin geliri instead of just şirket geliri?

This uses the genitive-possessive construction to show exactly whose income it is. Because we are talking about a specific company, şirket gets the genitive suffix -in (making it "the company's"). Then, the word gelir (income) takes the possessive suffix -i to complete the grammatical pair.

Is the verb azaldı related to the word az (little)?

Yes, it is! The root is the adjective az (little or few). Adding the suffix -al turns it into the verb azalmak, which means "to become less" or "to decrease". Finally, the -dı at the end is the simple past tense suffix, meaning "decreased".

Where is the word for "by" in this sentence?

Turkish doesn't use a separate word for "by" when talking about increasing or decreasing by a certain amount. The fraction onda bir simply sits directly in front of the verb azaldı to show how much the income changed.