Question 322700
Since it is decreasng by 20%, then 80% or 4/5 remains and we can write

{{{50(4/5)^t}}}, where t is the year.

So we do not have such large numbers to deal with, divide by a million and let the initial amount be 50 and then 100000 would be 1/10.

Now, set this equal to 1/10 and solve for t.

{{{50(4/5)^t=1/10}}}

{{{(4/5)^t=1/500}}}

{{{tln(4/5)=ln(1/500)}}}

{{{t=ln(1/500)/ln(4/5)}}}

t=27.85 years