T1/2 = ln2/lambda where T1/2 is half-life and lambda is the decay constant.
Also A = lambda x N where A is activity, lambda is the decay constant and N is the number of nuclei.
To calculate the amount of a sample remaining after a certain amount of time use the formula 1/2^n where n is the number of half-lives.
To calculate the amount that decayed you can use 1 - 1/2^n since its 1 minus the amount remaining.
Hope this helps.