Answer:
12 minutes.
Step-by-step explanation:
A water hole can fill a pull in 30 minutes and a larger hole can fill the same pull in 20 minutes.
Therefore, the small hole can fill in 1 minute the [tex]\frac{1}{30}[/tex] part of the pull and the large hole can fill in 1 minute the [tex]\frac{1}{20}[/tex] part of the pull.
So, if both the holes are open within 1 minute they will fill the tank by [tex](\frac{1}{20} + \frac{1}{30}) = \frac{1}{12}[/tex] part of the pull.
Hence, the total pull will be filled within [tex]1 \div \frac{1}{12} = 12[/tex] minutes. (Answer)