Solution (source code)

= Solution

The numerator event is contained in the conditioning event. The definition of <conditional probability> therefore gives
$$
\boxed{P(1<X<3\mid1<X<6)
=\frac{P(1<X<3)}{P(1<X<6)}
=\frac{1-3^{-\alpha}}{1-6^{-\alpha}}}.
$$
The denominator is positive for all $\alpha>0$, and endpoint strictness is irrelevant for a <continuous probability distribution>.