Set this up as
10 = 40 (1/2)t/28
and solve for t in years.
10/40 = (1/2)t/28
log(0.25) = (t/28) log(0.5)
t = 28 log(0.25) / log(0.5) years = 56 years
Test the answer:
56 years = 2(28 years) = 2 half-lives, so the remaining amount should be 1/2 of 1/2 of the initial amount, i.e. 1/4 of the initial amount, and 10 mg = 1/4 of 40 mg.
There’s no picture attached.
Mesosphere is the layer after stratosphere, which is after troposphere, as seen from Earth in the direction of Space.
It starts at 60 km and ends at 120 km, after which comes the termosphere.
So we're looking for a line that's closest to 100 km, and this is the shortest line, A.
