What is the average price per share for John's holdings after Date 4?

[vertciel]

Dear all,

Although the following question involves some terms using finance, I post it here since it seems to involve some probability. I hope that it will be fine.

Thank you very much for your help.

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Problem: Suppose that John buys shares in only one company and he does so as follows:

Date 1. He buys 10 shares @ $10.
Date 2. He buys 5 shares @ $20.
Date 3. He sells 6 shares @ $15.
Date 4. He buys 10 shares @ $20.

After Date 4, what is the average price per share for John's holdings?

Attempt at Solution :

Before Date 3, the weighted average price is given by:

$\frac{10}{15}10 + \frac{5}{15}20$

and the total amount before Date 3 is just $30.

At Date 3, John sells $6 \times 15 = 90$ of this stock

$\Rightarrow$ Total stock holdings = $200 - 90 = 110$.

Now, this is where I am having trouble. To calculate the average price, it appears that I would need to identify specifically the purchase dates of the 6 shares sold at Date 3, since Date 1 and Date 2 involve different purchase prices.

Is this correct? If so, how would I continue my calculation if I were to assume that the 6 shares sold were randomly chosen from the 15 shares from Date 1 and Date 2?

 

[mathman]

For tax purposes, he needs to identify. However as your calculation shows, it doesn't matter otherwise. In fact you can simply add up his purchases, 25 shares for $400 and his sales 6 shares for $90, leaving 19 shares for $310.