Measurement operators

What is the probability of obtaining the result |0> when measuring the following state?

$|\psi\rangle=\sqrt{\frac{1}{3}}|0\rangle+\sqrt{\frac{2}{3}}|1\rangle$

The answer is:

$P_0=|\langle 0 | \psi \rangle |^2=\left | \sqrt{\frac{1}{3}}\right|^2=\frac{1}{3}$

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