Let Q(t) denote the population of the world at time t. In 1999, the world population was 6.0 billion and increasing at 1.3% per year; in 2009, it was 6.8 billion and increasing at 1.2% per year.
In which year, 1999 or 2009, was dQ/dt (the rate of growth of Q with respect to t) greater? Explain.