Question

The radioactive substance Mathium has a half-life of 1 minute. If we have 32 pounds of Mathium at the start, how long will it take us to have only 4 ounces of Mathium left?

• A. 6 minutes
• B. 7 minutes
• C. 8 minutes
• D. 9 minutes

Answer 1

The correct option is B 7 minutes

16 oz = 1 lb
4 oz = 1/4 lb
Mathium becomes half after every minute.
$32\to 16\to 8\to 4\to \to 2\to 1\to 1/2\to 1/4$
After 7 minutes, Mathium would be 4 oz or 1/4 lb.

Using exponents, we get $32\times \frac{1}{2^7}=32\times \frac{1}{128}=\frac{32}{128}=\frac{1}{4}$.