The relational algebra expression equivalent to the following tuple calculus expression:
$\left\{t \mid t \in r \land \left(t[A] = 10 \land t[B]=20\right)\right\}$ is
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$\sigma_{(A=10 \lor B=20)} (r)$
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$\sigma_{(A=10)} (r) \cup \sigma_{(B=20)} (r)$
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$\sigma_{(A=10)} (r) \cap \sigma_{(B=20)} (r)$
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$\sigma_{(A=10)} (r) - \sigma_{(B=20)} (r)$