At the end of the 4th half-life, how much of a material will have decayed?

93.75% of a material decays at the end of half-life.

Explanation:

            The time needed by a material to reduce its initial amount into half is called half-life. It is generally used term in nuclear physics. This is to explain how fast unstable atom undergoes, or how long can stable atoms present, and radioactive decay.  

            Decay % of material after each half-life to be calculated as follows,  

After one half-life- 50% original, and 50% product decayed.  

After two half-life- 25% original, and 75% product decayed.  

After three half-lives- 12.5% of original, and 87.5% of decay product.  

After four half-lives- 6.25% original, and 93.75% of decay product.  

f=ma newton

f=100x2.5=250n