Radium-221 has a half-life of 30 seconds. How long will it take for 80% of a sample to decay

70 seconds (approximately)

Step-by-step explanation:

Let X be the initial amount

80% decay, 20% remaining

0.2X = X (0.5^t)

0.5^t = 0.2

t (lg0.5) = lg (0.2)

t = lg (0.2) : lg (0.5)

t = 2.3219280949

2.3219280949*30

= 69.65784285

approximately 70 seconds