A number decreased by 12 is 24.
(by looking at your question, we can see that 'decreased by 12' = subtract (-) 12)
We can write this as the equation n - 12 = 24, in which n = the number.
Now let's solve for n by using algebraic properties.
n - 12 = 24
n - 12 + 12 = 24 + 12 (add 12 to both sides to isolate n)
n = 24 + 12 (the -12 and +12 cancel to 0 on the left side)
n = 36 (simplify, add 24 and 12)
Therefore the number is 36.
