Quick questions on Simple interest graphs for HSC Maths Standard 2

To draw the graph, first simplify the formula for your principal and rate so only $n$ is left, then make a short table of values, plot the points, and join them with a ruled straight line. For $10\,000 at $4\%$ per annum, $I = 10\,000 \times 0.04 \times n = 400n$, giving the table

The most common exam task is comparing rates. It is built up in stages below: draw the axes, plot the lower rate, add the higher rate, then read off. Both deposits below are $10\,000. The only difference is the rate, $4\%$ versus $6\%$.

Some questions plot the amount $A$ (the value of the investment, or total owed), not the interest. The shape is the same straight line with the same gradient $Pr$, but it is lifted up by the principal: at $n = 0$ the account already holds $P$, so the line starts at $A = P$ on the vertical axis, not at the origin. For $8000 at $5\%$ per annum the gradient is $8000 \times 0.05 = 400$ and the intercept is $8000, so $A = 8000 + 400n$. Reading up at $4$ years gives $9600.