八十四 (hachi-juu yon) would be "84" in Japanese. Literally what is being said is "eight-ten four" and you may realize from this that the Japanese actually use this stylistically throughout the rest of their numbering system. So from here it should be easy to see why 九十七 (kyuu-juu nana) is "97" or 三百五十六 (san-byaku go-juu roku) is "356".
81+0=81
81/100
81,000 is 81 thousands.0.081 is 81 thousandths.
It is: 81 = LXXXI
81 m2
81/100
81% as a decimal is 0.81
80+1= 81
81 = 8.1 x 101
92 = 81 AND 3^4
81/100
The square root of 81 is 9