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Old 05-23-2013, 06:49 PM   #1
Lord Morpheus
 
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Default 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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Old 05-23-2013, 07:15 PM   #2
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Default Re: Power Stone Price

Quote:
Originally Posted by Lord Morpheus View Post
Do someone knows what happens? Am I doing something wrong?
You're missing "Powerstone prices account for this risk; hence, the true materials cost is divided by 53/54 to the power of P."
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.
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Old 05-23-2013, 08:56 PM   #3
Lord Morpheus
 
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Default Re: Power Stone Price

Quote:
Originally Posted by Anthony View Post
You're missing "Powerstone prices account for this risk; hence, the true materials cost is divided by 53/54 to the power of P."
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.
Thanks I was hopping someone will get this in the discussion. I really don't understand this part of the math. (54/53)^50
What is the calculation here. Can you make this step by step?
Thanks again (my math abilities are a disgrace)
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Old 05-23-2013, 11:03 PM   #4
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Default Re: Power Stone Price

Quote:
Originally Posted by Lord Morpheus View Post
Thanks I was hopping someone will get this in the discussion. I really don't understand this part of the math. (54/53)^50
What is the calculation here. Can you make this step by step?
Thanks again (my math abilities are a disgrace)
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.
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Old 05-23-2013, 11:15 PM   #5
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Default 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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Old 05-23-2013, 11:16 PM   #6
Anthony
 
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Default Re: Power Stone Price

Quote:
Originally Posted by Flyndaran View Post
I don't like Magic's assumption that every one of the inexplicably common enchanters has over 20 skill with whatever they're enchanting.
It's irrelevant to powerstones; you only need a skill 16 master.
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Old 05-24-2013, 09:33 AM   #7
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Default Re: Power Stone Price

Quote:
Originally Posted by Lord Morpheus View Post
Thanks I was hopping someone will get this in the discussion. I really don't understand this part of the math. (54/53)^50
What is the calculation here. Can you make this step by step?
Thanks again (my math abilities are a disgrace)
You take 54 and divide it 53 getting1.01886792
Then multiplying the above answer by it self 50 times getting an answer of 2.54621114
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Old 05-24-2013, 02:07 PM   #8
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Default Re: Power Stone Price

Quote:
Originally Posted by chimchim View Post
You take 54 and divide it 53 getting1.01886792
Then multiplying the above answer by it self 50 times getting an answer of 2.54621114
Thanks, now I get it!
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Old 05-24-2013, 02:31 PM   #9
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Originally Posted by Lord Morpheus View Post
Thanks, now I get it!
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 1-3 quirks.
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