How number bases work
Decimal uses ten digits. Binary uses only 0 and 1, which is how computers store everything. Hexadecimal uses 0 to 9 and A to F, so each hex digit stands for exactly four binary digits; that's why colours and memory addresses are written in hex. Prefixes such as 0x, 0b and 0o are understood.
Roman numerals
I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1000. A smaller numeral before a larger one is subtracted, as in IV for 4 and CM for 900. Standard Roman numerals go up to 3999 (MMMCMXCIX).
Writing numbers in words
British English puts “and” after hundreds, as in one hundred and five; American English often leaves it out. The check style writes dollars in words and cents as a fraction, such as One thousand two hundred fifty and 50/100.