Step 1 :Let the tens digit be x and the units digit be y. We know that \(x = y - 1\) and the number itself can be represented as \(10x + y\). The sum of the digits is \(x + y\).
Step 2 :We are given that the sum of the digits \((x + y)\) is equal to one-fifth of the number itself \((10x + y)\). So, we can write the equation: \(x + y = \frac{1}{5} (10x + y)\).
Step 3 :Solve the equation to find the values of x and y: \[\begin{cases} x = y - 1 \\ x + y = 2.0x + 0.2y \end{cases}\]
Step 4 :The solutions are \(x = 4\) and \(y = 5\).
Step 5 :\(\boxed{45}\) is the two-digit number.