y= x/200+400 as a check insert the given information of $5k spent and see if it matches. y=5000/200+400. y=25+400, ( the 25 from the $5k spent, and the 400 that they sell with zero spent )
If the firm spends $25k on advertising it will sell 525 units. if each $5k spent generates 25 sales, then $25 k can be broken down as $5k x 5. Each $5k generates 25 sales, so 5 times that will generate 25 x 5, or 125. Plus the 400 units it sells without advertising, = 525 ( 125 + 400 )
If the firm spends $50k it will sell 650 units. The 400 that it sells anyway, plus 10 times when it sells for spending $5k, or 250. 250+400=650
To sell 700 units we first subtract the 400 it sells anyway, then divide 25 into the remainder. So we need to sell 300 units. Each 25 units costs $5k. 300/25=12 so we will have to spend 12 x $5k or $60k.