half-life

What are the answers you've got? And is that the exact wording of the question? Half-life for a given isotope is the same, no matter how much time has passed. Is the question asking for the activity of the sample after five minutes?

Some definitions:
Half-life- the time it takes for the activity of a radioactive sample to fall by half.
Activity- the amount of radiation emitted by a radioactive sample in a given time-frame. 1 Bequerel is equivalent to one unstable nucleus decaying per second.
The question asks for the activity of the sample after five minutes. This does not mean anything like the 'half-life of the half-life'. Activity is distinct from half-life, but knowing the half-life and initial activity allows you to calculate the activity at a given time.
So, to calculate the activity after any given length of time, you first have to determine how many half-lives have passed. Do you know how to work that out?

no, I don't. thanks for clarifying the definitions as well

Have you done any questions on binary fission and mean division time in biology? If you have, it's a bit like that. If not, don't worry.
The question says that the half-life is 10 minutes, and you have to work out the activity after five minutes. This is an odd question, because usually the given half-life is shorter than the time which has passed.
What you have to do is to work out how many times the half-life time has passed. In other words, how many times does the half-life occur within the given period.

In this case, 5/10=0.5, so 0.5 half-lives have occurred.
If one half-life had passed, then the initial activity would have fallen by half, but since only half a half-life has passed, the activity has dropped by a quarter (meaning that the activity is now 3/4 what it was originally, not 1/4). You can work out the answer from this and the initial activity.

Another example:
If, say, the question asked for the activity after 20 minutes, and the half-life was still 10 minutes, the method is the same. 20/10=2.
In this case, the activity would have halved twice. With an initial activity of 100Bq:
After first half-life: 100/2= 50Bq
After second half-life: 50/2= 25Bq

Do you have the answers, or is it an online thing which just tells you if you're right or wrong?

That makes sense, though with that, I don't see how to find the answer without drawing a graph, sorry.

How? Did you guess, or find a method?
And no problem