Answer:
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
