after two half - lives, how much of the original material has decayed?\n25 percent\n50 percent\n75…

Explanation:

Step1: Define half - life concept

The amount of a radioactive substance remaining after one half - life is $\frac{1}{2}$ of the original amount.

Step2: Calculate remaining amount after two half - lives

After the first half - life, the remaining amount is $\frac{1}{2}$ of the original. After the second half - life, the remaining amount is $\frac{1}{2}\times\frac{1}{2}=\frac{1}{4}$ of the original.

Step3: Calculate decayed amount

The decayed amount is $1-\frac{1}{4}=\frac{3}{4}$. Converting $\frac{3}{4}$ to percentage, $\frac{3}{4}\times100 = 75%$.

Answer:

C. 75 percent