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Answer: 90 is a number which has 9 as the sum of its digits, 9+0=9 and if the digits are reversed, 09 is 81 less than 90.
Step-by-step explanation: Since the number must be a 2-digit number, it seems that 90 is the only number that qualifies.
If you need some equations, here is a way to set up a system to solve:
x+y= 9 rewrite to get a value for y to substitute:
y = 9-x
10x +y = 10y + x + 81 rewrite by moving (subtracting variable terms from both sides)
10x -x + y-10y = 81 combine like terms
9x - 9y = 81
Substitute for y and solve for x
9x - 9(9-x)= 81 distribute.
9x - 81 + 9x = 81
18x = 162. 162/18 = 9
x = 9
Substitute in the original equation to solve for y
9 + y = 9
y = 0
10(9) + 0 = 10(0) + 9 +81
90 - 81 = 09