Floor function equation

Algebra Level 2

$\big\lfloor 5 - \lfloor x \rfloor \big\rfloor = 15$

If all the values of $x$ that satisfy the equation above are in the interval $a \le x < b$ , find the product $ab$ .

Note: $\lfloor x \rfloor$ is the floor function, or the greatest integer function.