The answer is
The exponential decay of an element can be written as
Therefore, we get
It will take 0.05s.
A quick way is to count the number of Half-Lives which have elapsed. To go 1/2 then 1/4 of the original amount = 2 Half-Lives. So time elapsed = 2 x 0.025 = 0.05 s.
If the numbers don't work out so nicely you will need to use the method described in Stefan's answer.