
05232013, 06:49 PM  #1 
Join Date: Nov 2005
Location: Brazil

Power Stone Price
Please if some on cam help, I will be glad.
In the GURPS Magic, p.20, we find a equasion to calculate the cost of a powerstone. The material cost is calculate like this: (10xP˛)+(40xP)  Where P is the powrstone capacity of the powerstone. The labor cost is 20xP Ai meke the calculations for every stone in the table, and just the first one is correct. The other ar wrong, at start with a small diference, as de2 pt Powerstone, the cost in the book is 165, but if we use the formulas we find 160. Until the 6x more expensive in the last on (100), the cost in the book is 675.000, and if we use the formula that results in 106.000. Just tu make a exemple, lats get de 50 Energy point stone. Material (10xP˛)+(40xP)= $ (10x50˛)+(40x50)= $ (10x2500)+(2000)= $ (25000)+(2000)= $ $=27.000 Labor 20xP=$ 20x50=$ $=1000 Material+Labor= 27.000+1000=28.000 Total 28.000 => cost in the book 69.5000! Do someone knows what happens? Am I doing something wrong?
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05232013, 07:15 PM  #2  
Join Date: Feb 2005
Location: Berkeley, CA

Re: Power Stone Price
Quote:
The actual cost of a 50 point stone is: materials: (40 * 50 + 10 * 50 * 50) * (54/53)^50 = 68,745 labor: 20 * 50 = 1,000. Total 69,745, which inexplicably got rounded to 69,500. Note that the box on Magic 20 assumes that quirks don't influence the value of powerstones, which is unrealistic, and ignores some more complex math for the actual number of castings required to produce a powerstone. 

05232013, 08:56 PM  #3  
Join Date: Nov 2005
Location: Brazil

Re: Power Stone Price
Quote:
What is the calculation here. Can you make this step by step? Thanks again (my math abilities are a disgrace)
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05232013, 11:03 PM  #4 
Join Date: Feb 2005
Location: Berkeley, CA

Re: Power Stone Price
There is a 4/216 (1/54) chance per try to blow up the power stone, meaning there's a 53/54 chance per try to survive the attempt; 50 tries means (53/54)^50 chance of survival (because a failed roll doesn't add a mana point, it should actually be 206/210 ^ 50). Thus, you need to adjust the cost for the chance of blowing up your stone.

05232013, 11:15 PM  #5 
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Join Date: Oct 2004
Location: Forest Grove, Beaverton, Oregon

Re: Power Stone Price
I don't like Magic's assumption that every one of the inexplicably common enchanters has over 20 skill with whatever they're enchanting.
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05232013, 11:16 PM  #6 
Join Date: Feb 2005
Location: Berkeley, CA

Re: Power Stone Price

05242013, 09:33 AM  #7  
Join Date: Jan 2005
Location: South yorkshire, united kingdom

Re: Power Stone Price
Quote:
Then multiplying the above answer by it self 50 times getting an answer of 2.54621114
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05242013, 02:07 PM  #8 
Join Date: Nov 2005
Location: Brazil

Re: Power Stone Price
Thanks, now I get it!
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05242013, 02:31 PM  #9 
Join Date: Feb 2005
Location: Berkeley, CA

Re: Power Stone Price
Note that, because failed results don't actually add to the enchantment on the stone, the correct ratio is 210/206, not 216/212, meaning the actual multiplier for a 50 point stone is 2.6157 and the final stone cost is $71,625. Except that you also want to know about quirks, which we can approximate as a binomial distribution where n is the size and p is 6/212 (it's not actually a true binomial); for a 50 point stone, of the surviving stones, roughly 23% will be unquirked, the rest will mostly have 13 quirks.

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