Explanation:
that is exactly what a function and its graph is about.
one variable (y) changes completely dependable on the changes of another variable (x).
by changing x we can calculate our see in the graph the consequences in y.
now, here y is renamed to "value", and x is renamed to "time".
so, we see how the value of the washing machine decreases over time. the older the machine the smaller its value.
as the graph is a straight line we call this a linear function.
that means for the whole function the ratio y/x is always a constant value. that is the slope.
1.
the slope is defined as (y coordinate difference / x coordinate difference) when going from one point on the line to another point on the line.
what points can we see on the line ?
e.g. for time (or x) = 0 we get value (or y) = 500.
this is the point (0, 500).
for time (or x) = 1 we get value (or y) = 400
this is the point (1, 400).
x changes by +1 (from 0 to 1).
y changes by -100 (from 500 to 400).
the slope is
-100/+1 = -100
it means for every additional year of age the value goes down by 100.
2.
the y-intercept is at the point (0, 500), and it means that at age 0 (brand new machine) it value is 500.
3. the x-intercept is at point (5, 0). that means when reaching 5 years of age (and then beyond) the value of the machine is 0. you cannot expect to resell it or have an insurance company pay for repairs.
4.
there is no specific interval. the continuous line means that the machine loses value continuously. not just every year, but every month, every week, every day, every hour, every minute.
but as the slope tells us, with every year the loss in value is 100. this ratio is constant everywhere and for every interval you look at. that means with every fraction of 1 year, the machine loses the same fraction of 100. or with every multiple of 1 year, the machine lose the same multiple of 100 as value.