Question

Uranium-235 has a half-life of 3 minutes. A fresh sample weighing 80 pounds was obtained. If it takes 15 minutes to set up an experiment using Uranium-235, how much Uranium-235 will be left when the experiment begins?

• A. $0.25$ pounds
• B. $2.5$ pounds
• C. $25$ pounds
• D. $80$ pounds

Answer 1

The correct option is B $2.5$ pounds

Time (min)	Uranium-235 Remaining (Pounds)
0	$80$
3	$\frac{80}{2}=40$
6	$\frac{80}{2^2}=20$
9	$\frac{80}{2^3}=10$
12	$\frac{80}{2^4}=5$
15	$\frac{80}{2^5}=2.5$

$\therefore$ After $15\text{mins}$ i.e $5$ half life reamining Uranium-235 will be $\frac{80}{2^5}=2.5\text{pounds}$