Maths behind sliding scale pricing [closed]

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I'm trying to work out a simple calculation for the following:

a phone model has the maximum sale price of £85.00 and this is if only 1 unit is purchased and a minimum sales price of £50.00 - this is if 150 units and over are purchased in one.

How can I work out a way of the price if between 2 and 149 units are purchased??

Thanks, B.

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Formula:

Y = 50 + ((85 - 50) / (150 - 1)) * (X - 1)

Result:

X = 1    -->   Y = 85
X = 33   -->   Y = 57.52
X = 150  -->   Y = 50

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You probably don't have a linear model in such situations. I don't know the right English term, but you would have more like a piecewise or stepwise function here.

But nevertheless, you can get a linear function through interpolation.