Your book is correct. The number 100 is written as a one followed by two zeroes. Many one-syllable nouns from Old English that end in F change the F to a V in the plural: wife — wives wolf — wolves knife — knives In Old English, the F and V were basically the same phoneme, and we only began to separate them after the Norman Conquest. Note that the plural of “chief” is “chiefs” (it’s a French word).

1. One zero, two zeroes When will we use things like that "one zero or two zeroes"? I see these in my grammar book but idk what are they for. Are you asking when to use double-quotes (" ")? The Internet is a powerful tool. Search for "punctuation rules double quotes". http://www.grammarbook.com/punctuation/quotes.asp (Please NEVER use "idk" for 'I don't know'. It makes me think you are very, very, very lazy. And it is NOT an appropriate abbreviation. No one will know what it means.) 2. Life - lives I always thought plural noun of life would be "lifes." But my book writes "lives." Is it correct? Yes The dictionary is a powerful tool. Learn to use it and you can answer these simple questions yourself. The dictionary will tell you the plural form of all nouns. http://www.merriam-webster.com/dictionary/life life => lives knife => knives wife => wives