11.8 hours ago, there were 70% more of the substance if an isotope of lead has a half-life of 8.4 hours.
What is half-life?
Half life is the time that it takes for half of the original value of some amount of a radioactive element to decay.
Half-life period - 50% -8.4hours
so for 10%- 8.4/5 = 1.68 hour
now for 70%
1.68 × 7 = 11.76 hours
To one decimal place - 11.8hours
Hence, 11.8hours is the answer.
To learn more about half-life here
https://brainly.com/question/17159063
#SPJ2
