The question states:

"The force of interest at any time \(t\), (measured in years) is given by

\(f(t) = \begin{cases}0.07-0.005t & 0 \le t < 5 \\ 0.06-0.003t & 5 \le t < 10 \\ 0.03 & t \ge 10 \end{cases}\)

What is the total accumulated in value at any time \(t > 0\) of investments of R100 at times 0, 4 and 6?"

I understand getting to first 'time period' i.e. for \(0 \le t \lt 4\) but from there on I get confused. The memo splits the answer into \(0 \le t \lt 4\), \(0 \le t \lt 5\), \(0 \le t \lt 6\), \(0 \le t \lt 10\), \(t \ge 10\). Why is it not split from 0 to 4, 4 to 5, 5 to 6, 6 to 10 and greater than 10?

reshown Apr 10, 2016 by simon_rigby

I think there was a mistake in that memo hey. The way you understand it is correct.