Numbers should be so easy. What is good from one to ten also works from 21 -30 or 81-90. Of course the 11-16 period is rough, just like teenagers. The period ends eventually and does not return. In other words, as Spock would say, numbers are logical.
The truth is that they are not, at least in many languages. The worst offender in my mind is French, which requires advanced math skills to figure out a phone number. If someone’s phone number is 15 63 78 97, it comes out: quinze, soixante trois, soixante dix huit, quarter- vingts dix- sept. That comes out as fifteen, sixty three, sixty ten and eight, and four twenties ten and seven in English. The first is a teen and the second is normal. However, the third requires me to practice addition while the fourth is for math majors, involving both multiplication and addition. Even more absurd, the French make of the Belgium for saying septante and nonante for seventy and ninety, eliminating all that calculation.
The Russian language also has its peculiarities. First, all numbers in a series, even if that number is inside a number as пятьдесят [pidicyat], where пять is five and десят is ten, i.e. fifty, must be changed to the correct case, such as пятидесяти [pitidecyati]. Even more curious is that the case of the noun following any numbers is determined by the last number only, ignoring all previous numbers. This creates the following logical absurdity in translation: I have three hundred thousand, four hundred and twenty one reader of my blog. The word reader is singular because the last number is one. By the way, it is not true about the actual current number of readers.
Hebrew has an interesting variation in numbers. Letters are often used as numbers, especially in religious documents and the Hebrew calendar. The letter י [yud] represents the amount of ten while the letters ה [heh] and ו [vav] represent five and six, respectively. When referring to the 15th and 16th day of a month or verse, the logical combinations are not used. Instead, טו and טז [nine and six / seven] are written. The reason for this is that יה and יו are abbreviations for God.
So, as any student of algebra knows, numbers are not just a matter of abc.