56, and that is only the magor volcanos inIndonesla
Because of Dhaka's position below sea level the city experiences yearly flooding, particularly around the monsoon seasons and has the constant threat of rising sea levels, which would of course cause more flooding.
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.
It is when warm air rises then cools then drops then rises then cools then drops etc
