Well, start from 100% and work backwards.
It takes 5600 years to get to 50%
Another 5600 years to get to 25%
And a bit under another 5600 to get to 15% (another 5600 years would get it down to 12.5%)
Add the first two 550s to get 1120, and add about 5,000 and see which answer is closest.
In this case, D is closest at 15!w29
There are more exact ways of solving, but for a multiple choice this is faster and efficient nonetheless