Usually it would be a good idea to get rid of it after the second week. 1/2 the value lost is very hard to make up. The word every means that the stock is going to loose it's 1/2 it's value in the first week. The formula is
y = 125* (1/2)^4
y = 125 / 16
y = 7.81
That means that at the end of the 4th week, it is worth just 7.81 dollars.
If that is not the intent and the first week does not count then the formula becomes
y = 125 * (1/2)^(n-1)
y = 125 * (1/2)^3
y = 15.625 still a pretty severe loss.