Which one is right for roman 51 LI or XLXI?

Answer 1

LI (50 + 1) is the accepted form of writing 51. XLXI would represent 40 + 11 and it is a totally incorrect and unnecessarily complicated way to write 51. It would not be considered good practice to deduct 10 from a number (XL) only to add 11 (XI) to it in the same numeral.