The equation to solve is P = P0e^kt. Where P = population, P0 = initial population, k is a constant and t = time.

To find k:

1990 = p0e^0k

-------------------

2000 = p0e^-10k

Once I divide these out I get 1990/2000 = e^10k.

To find k take the natural log of both sides:-

ln 1990/2000 = 10k. Therefore k = 1/10*ln 1990/2000.

To solve for the half life:

1000 = 2000e^(1/10 ln 1990/2000)t.

When I work through this I get:-

t = 10* (ln 1/2)/(ln (1990/2000)).

However the correct answer is:-

t = 10* (ln 2)/(ln (2000/1990)).

Much appreciated if someone can show where I am going wrong.