I've tried to simplify the following example as much as possible for ease of reading...

A manufacturer finds that for a new model, at £240 a unit, 20 units per week are sold. However, during promotion, the model was priced at £160 per unit, 45 units per week were sold. LetSbe number of units sold per week, andX(in GBP) be the price of a unit. All production is sold.

How do I show that, for value of the priceX, the number soldSis given

by the linear function:

S = 95 - (5/16)X

Any help much appreciated, Thank you.