Okay, here we have this:
Considering the provided information, we are going to calculate the requested value, so we obtain the following rule of three:

Solving for x:

Finally we obtain that 87.5mL of water should be used for 250g of plaster.
To overcome and identify outliers, the data obtained can be arranged into from the highest to lowest and be oberved on which values have the highest or lowest value.
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