Use differential approximations in the following problem. A company will sell N units of a product after spending $x thousand in advertising, as given by N = 70x-x, 5x35. Approximately what increase in sales will result by increasing the advertising budget from $15,000 to $16,000? From $20,000 to $21,000? Find the differential dN. dN = dx The increase in sales from increasing the advertising budget from $15,000 to $16,000 is approximately units. (Type a whole number.) The increase in sales from increasing the advertising budget from $20,000 to $21,000 is approximately units. (Type a whole number.)