Answer:
84%
Step-by-step explanation:
The first step is to write 21 as a fraction of 25, which is simply
21/25
Now we just need to convert this fraction to a percentage :
21/25 = 84
So, 21 is 84% of 25
I don't Clearly understand your question but I think it is 0.428571
The formula for half-life is:

A is the amount remaining, P is the initial amount, t is the time that has passed, and h is the half-life of the substance. Plug in the values that you know and solve:

Rounded to the nearest whole number, the density of the substance was about 13<span>mg / cm².</span>