What is wrong with my answer?

Question

The question asks me to calculate area under the graph. Seeing as gradient = -1, the x intercept is 60. The area under the graph would be 360, but this is wrong. Where is my mistake?

Answer 1

Check the scales: each vertical line is $5$ units apart, and each horizontal line is only $1$ unit apart. This makes the gradient $-\frac{1}{5}$. Splitting into a triangle and rectangle, I make the area to be: $$25 \times 1 + \frac{1}{2} \times 25 \times 5 = 87.5 \text{ units}^2.$$

Answer 2

You have different scales for $x$ and $y$. With equal units it seems as here:

As you can see, the gradient is not $-1$, but $-{1 \over 5}$.

Answer 3

The area under the graph is $360$, if you mean that it is bounded by $x$-axis.

But in your picture it is bounded by lines $y=6$, $x=0$, and $x=25$, so the area in your picture is $87.5$. Maybe your task was to determine this area.