We write the statement in algebraic form to solve the problem:
We name the unknown number "x" in our mathematical expression.
"8 less than a number is the same as one less than twice a number"
x - 8 = 2 x -1
"8 less than a number" is written as the unknown number minus 8
and
"one less than twice a number" is written as 2 x - 1
So now we solve for the unknown number in the equation we wrote:
x - 8 = 2 x - 1
We group all the terms with x on the right, and all numerical terms on the left of the equal sign:
-8 + 1 = 2 x - x
-7 = x
Therefore the unknown number is -7 (negative seven).