<PRE><B>Question 1096997</B>
&lt;br&gt;Tutor jorell380 is almost certainly a scientist more than a mathematician; scientists like to work exponential functions using e as the base.  For me, as a mathematician, working half life problems using powers of 1/2 makes far more sense.&lt;br&gt;
For problems with half lives, the exponential decay function is
{{{y = A*((1/2)^x)}}}&lt;br&gt;
y is the amount remaining; A is the initial amount; and x is the number of half lives.&lt;br&gt;
For your problem, we have 20 zulus to start and 5 at the end.  So the number of half lives is found by solving the following equation for x:&lt;br&gt;
{{{5 = 20((1/2)^x)}}}
{{{1/4 = (1/2)^x}}}
We don&#39;t need any logarithms there! 1/4 is (1/2)^2; it takes 2 half lives for the number of zulus to drop from 20 to 5.&lt;br&gt;
The half life is 14 days, so 2 half lives is 28 days.&lt;br&gt;
Answer: 28 days</PRE>