十(shí) - ten in Mandarin

十 (shí) is ten, and it is the hinge of the whole number system. Once you have 一 (yī) to 十 you can count to ninety-nine without learning another word. Eleven to nineteen are ten plus a unit: 十一 (shíyī), 十二 (shí'èr), and so on up to 十九 (shíjiǔ). The tens are a unit times ten: 二十 (èrshí, twenty), 三十 (sānshí, thirty), 九十 (jiǔshí, ninety). Put the two together and 二十五 (èrshíwǔ) is twenty-five, which is as logical as counting gets.

Two details. On its own or at the front of a number, ten is normally plain 十, not 一十, so fifteen is 十五 (shíwǔ). After 百 (bǎi), though, the 一 comes back: 110 is 一百一十 (yìbǎi yìshí), not 一百十. And in the middle of a longer number 十 is usually said lightly, close to a neutral tone, so 二十五 comes out nearer èrshi wǔ in ordinary speech. The dictionary tone is still a rising shí, and that is what you should write.

Do not confuse the number with 十分 (shífēn), an adverb meaning very or extremely, as in 十分好 (shífēn hǎo, extremely good). It has nothing to do with ten any more. The trap is that 十分钟 (shí fēnzhōng) is ten minutes, so the same two characters can be either: look at what comes after them before you decide.