'e' is Euler's Number. It is a constant, just like pi, and has a value of 2.718... you can read about about it at
http://mathforum.org/dr.math/faq/faq.e.html
You won't typically use the number in calculations, but instead as part of a "natural logarithm", which is Log[sub:3om6s84d]e[/sub:3om6s84d], typically written as LN.
P[sub:3om6s84d]0[/sub:3om6s84d] is a constant as well, in this case the starting population. Because your problem doesn't have a specific value for the population, you can just use P[sub:3om6s84d]0[/sub:3om6s84d]. You could use any letter, and the 0 just means it's the initial value.
Now, your problem looks like this:
3*P[sub:3om6s84d]0[/sub:3om6s84d]=P[sub:3om6s84d]0[/sub:3om6s84d]*e[sup:3om6s84d].025t[/sup:3om6s84d]
t is the value you need to solve for. In order to solve for an unknown in an exponent, you need to rewrite this problem as a logarithm. Take the natural log of each side.
3*P[sub:3om6s84d]0[/sub:3om6s84d]=P[sub:3om6s84d]0[/sub:3om6s84d]*e[sup:3om6s84d].025t[/sup:3om6s84d]
3=e[sup:3om6s84d].025t[/sup:3om6s84d]
LN(3) = LN(e[sup:3om6s84d].025t[/sup:3om6s84d])
An exponent inside of a logarithm can be brought outside and multiplied by the logarithm (it's just a basic property of logs):
LN(3) = .025t * LN(e)
Can you solve it from here?
