Key idea. Assume there is a hippo and follow the consequences.

1. The zoo is known to have a giraffe. Suppose it also has a hippo.

2. The rule about hippos and giraffes would then force it to have a rhino.

3. But a zoo with both a hippo and a rhino cannot have a giraffe. This contradicts the giraffe that is already there. So our assumption was impossible: there is no hippo.

4. Every zoo must have at least a rhino or a hippo. Since this zoo has no hippo, it must have a rhino.

5. A giraffe and a rhino, with no hippo, cause no conflict with any of the three rules.

Remember. A contradiction rules out the assumption that led to it.