There's a way to do this: The shop's income is the price multiplied by the amount of packs sold. First we need to find a term for the price: we'll use x for this, nothing complicated so far. The function for the price is f(x):x Next we need the function for the amount of packs sold. This is going to be a straight line, because the rate of change is constant (2 packs per 5c). We know that a point on this line is (0.45, 36). Since the amount of packs goes down by 2 per 5c increase, we know that the slope is -2/0.05, which is -40. Now, we get the equation of the line by using the equation y=-40x+c. Put in the values for x and y, and you get 36=-40(0.45)+c, which will eventually work out to c=54.
The function for packs sold is y=54-40x.
Now, we multiply our packs sold by our price to get this new equation:
x(54-40x)
f(x): 54x-40x^2
I hope that makes sense. If it doesn't, message me and I'll clear it up.
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