This is because 57 - 13 = 44. 44 divided by 4 = 11. This means that in 11 minutes Karen will have a 57 liter tank. But she could keep going, but the least number of minutes would be 11 minutes, to get to 57 liters.
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You can have a maximum of 3 groups. This is due to the fact that you only have 3 sopranos to equally divide between groups. Over 3 you would have a group lacking a soprano so 3 is the greatest you can make it.