title: Maths behind sliding scale pricing
body: <p>I'm trying to work out a simple calculation for the following:</p>
  
  <p>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.</p>
  
  <p>How can I work out a way of the price if between 2 and 149 units are purchased??</p>
  
  <p>Thanks,
  B.</p>