When you reverse the digits in a certain 2 digit number you decrease its value by 81. What is the number if the sum of its digits is 9?

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

f(x) = y

f(-3) = -5

step-by-step explanation: