What mass of a 60.0-gram sample of n-16 will remain unchanged after 28.8 seconds?

The answer is 3.75 g.

Half-life is the time required for the amount of a sample to half its value.
To calculate the fraction of the sample remained unchanged, 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 N-16 is 7.13.
So, we know:
t = 28.8 s
= 7.13 s

We need:
n = ?
x = ?

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

⇒ 
⇒ 
⇒ n ≈ 4

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

⇒ 
⇒

If the fraction of the sample is 1/16 = 6.25%, then the mass of the sample could be calculated as:
x g : 6.25% = 60 g : 100% 
x = 

It would be (at) + cu (s)