Answer: 7.81
The formula is
y = 125* (1/2)^4
y = 125 / 16
y = 7.81
So at the end of the 4th week, the stock is worth only 7.81 dollars.
If that is not the motive and the first week doesn't count then the formula changes to...
y = 125 * (1/2)^(n-1)
y = 125 * (1/2)^3
y = 15.625 could be the other answer if you looked at it in that perspective
However, I would go with 7.81 as the answer.
Hope I helped :)