What fraction of a sr-90 sample remains unchanged after 87.3 years

The answer is 1/8.

Half-life is the time required for the amount of a sample to half its value.
To calculate this, we will use the following formulas:
1. ,
where:
n - a number of half-lives
x - a remained fraction of a sample

2. 
where:
- half-life
t - total time elapsed
n - a number of half-lives

The half-life of Sr-90 is 28.8 years.
So, we know:
t = 87.3 years
= 28.8 years

We need:
n = ?
x = ?

We could first use the second equation, to calculate n:
If:
,
Then: 

⇒ 
⇒ 
⇒ n ≈ 3

Now we can use the first equation to calculate the remained amount of the sample.

⇒ 
⇒