You have to make an equation that relates the amount of water (w) in the aquafier to time (t).
Start with the original amount:
Now, we know that every day ten thousand gallons are taken out so we have:
And we also know that every day two thousand gallons are added so we have:
To find when the water runs out, set the equation equal to 0 and solve for t:
So, the water will run out after 125 days.