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.
For the rate of 25 pieces and hour it will take 7 hours.
500 - 325 = 175
175 ÷ 25 = 7
7 hours
7 yd is the diameter, so just divide 7 by 2 to get the radius which is 3.5 yd