Conditional variance problem on INTRODUCTION TO PROBABILITY(2nd edition)

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How can I get results in the red box?

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It is a well-known fact that the variance of a uniformly distributed random variable on the interval $(a, b)$ is given by $(b - a)^{2}/12$. This can be shown using the Law of the Unconscious Statistician.

So,

$$\text{var}(X \mid Y = 1) = \frac{(1)^{2}}{12} = \frac{1}{12}$$

$$\text{var}(X \mid Y = 2) = \frac{(2)^{2}}{12} = \frac{4}{12} $$