# Hello,I'm revising half-life for GCSE and have come across some

Hello,
I'm revising half-life for GCSE and have come across some questions regarding fractions.

For example, "what fraction of the original nuclei will still be unstable after 5 half-lives?"

Am I right in thinking it would be 1/20 as 1/2 x 5 = 1/20 or is that too simple?

Thanks,
Molly

SammyS
Staff Emeritus
Homework Helper
Gold Member

Hello,
I'm revising half-life for GCSE and have come across some questions regarding fractions.

For example, "what fraction of the original nuclei will still be unstable after 5 half-lives?"

Am I right in thinking it would be 1/20 as 1/2 x 5 = 1/20 or is that too simple?

Thanks,
Molly
Yes, that's too simple ... and how is (1/2) × 5 = 1/20 ?

After a time of one half-life, 1/2 of the original nuclei remain unchanged.

After a time of an additional half-life, 1/2 of that 1/2 of the original nuclei remain unchanged. This is a total time of two half-lives and 1/4 of the original nuclei remain unchanged.

Etc.

Yeah, thank you I just realised I was being stupid. It's 1/32 right?

SammyS
Staff Emeritus