Puzzle ZNBA 

Workings

Hint
How much of a warren each rabbit dig in one hour?

Answer
2 hours.

Reasoning
The small rabbit can dig a warren in nine hours, so digs ¹/₉ in one hour.

The medium rabbit can dig a warren in 6 hours, so digs ¹/₆ in one hour.

The large rabbit can dig a warren in four and a half hours, so digs ¹/_4.5 in one hour.

   1 Hour = ¹/₉ + ¹/₆ + ¹/_4.5

To make the denominators all the same, we can multiply through by ¹⁸/₁₈ to give:

   1 Hour = ²/₁₈ + ³/₁₈ + ⁴/₁₈

   1 Hour = ⁹/₁₈

   1 Hour = ¹/₂

So in one hour, they have dug half a warren, the whole warren will therefore take 2 hours.