How long does it take a 100.00g sample of As-81 to decay to 6.25g?

Time taken for the decay = 133.2 seconds
Half-life refers to the time required for a sample to get reduced to half its initial quantity,
In this case, initial amount of the sample is 100 g.
At zero time: 100 g
After 1st half-life period : 50 g
After 2nd half-life period : 25 g
After 3rd half-life period : 12.5 g
After 4th half-life period : 6.25 g
So, 4 half -life periods have lapsed during the radioactive decay.
Half-life period of As-81 is : 33.3 seconds
So, time taken for the decay = 33.3 X 4 = 133.2 seconds