Aim: To calculate the value of a deposit of gold to a guild bank (gbank) that discounts sales at a given rate.
Summary:
- A donation of G gold to a gbank with a decimal discount rate of r will be worth G/r gold. This means that G is multiplied by 1/r, the reciprocal of the discount rate. Keep in mind that Full price = Discounted price + Discount.
- As long as the discount rate is less than 100%, the donation is worth more than the amount donated (even in pure money terms)!
- A gbank donation's impact will grow according to the sum of a geometric series and approach a limiting value.
The given:
- The discount rate is fixed.
- There is no cost in performing transactions and the full proceeds of a gbank sale are deposited back in.
- All gold is used in buying other resources for resale. No gold is dispensed as a repair allowance (though it can be factored into the discount rate).
The detail:
Consider a
gbank deposit of
G gold. It is used in
buying (for example) enchanting materials worth
G gold. They are
sold to a guild member at a
discount rate of
r, so the
undiscounted proportion of the price is:
1 - r
The
proceeds from that sale are:
G × (1 - r) = G(1 - r) gold
The proceeds are then used for buying potions worth
G(1 -
r) gold. There potions are sold at a discounted price of:
G(1 - r) × (1 - r) = G(1 - r)2 gold
Continuing this process for
n terms, we get the following
progression of values:
G, G(1 - r), G(1 - r)2, G(1 - r)3, ... , G(1 - r)n - 1
The geometric series 1 + 1/2 + 1/4 + ... , with its value represented by area.This is in fact a
geometric progression with
common ratio (1 -
r) and
first term B. Note that, while the
amount deposited diminishes, it keeps making an
impact with each gbank sale. The
total impact for
n sales is the following
sum of the corresponding
geometric series:
S(n) = G(1 - (1 - r)n) ÷ (1 - (1 - r)) = G(1 - (1 - r)n)/r gold
As
n »
∞ , (1 -
r)
n » 0 , and (1 - (1 -
r)
n) » 1 . So, the total impact approaches the following
limiting value as resales are conducted repeatedly with that amount:
S(∞) = G/r gold
Here is an example. Say 100 gold is donated to a gbank with a discount rate of 25% or 1/4. That donation will be worth:
100 ÷ 1/4 = 100 × 4 = 400 gold