Answer
$10 \text{ hours}$
Work Step by Step
Let $x$ be the time to empty a full tank.
In terms of $1$ part/unit, starting with a full tank, being filled at the rate of $2.5$ hours at the same time it is being drained at the rate of $2$ hours, then
\begin{array}{l}\require{cancel}
1+\dfrac{1}{2.5}-\dfrac{1}{2}=\dfrac{1}{x}
.\end{array}
Using the $LCD=
10x
$ and the properties of equality, the equation above is equivalent to
\begin{array}{l}\require{cancel}
10x\left(1+\dfrac{1}{2.5}-\dfrac{1}{2}\right)=\left(\dfrac{1}{x}\right)10x
\\
10x(1)+4x(1)-5x(1)=10(1)
\\
10x+4x-5x=10
\\
x=10
.\end{array}
Hence, the time, $x,$ it takes to empty a full tank is $
10 \text{ hours}
.$
