College Algebra, Chapter 1, 1.6, Section 1.6, Problem 88

Suppose that a coffee merchant sells a customer 3lb of a certain coffee at \$6.50 per pound. The merchant's scale is accurate to within $\pm 0.03$lb. if an accuracy in the scale occurs, by how much could the customer have been overcharged or undercharged?

$\displaystyle \frac{\$ 6.50}{\text{lb}} (3- 0.0.3\text{lb}) < x < \frac{\$ 6.50}{\text{lb}} (3 + 0.03) \text{lb}$
$\$ 19.305 < x < \$ 19.695$

If the scale is accurate, then the price will be...
$\displaystyle P = \frac{\$ 6.50}{\text{lb}} (3 \text{lb}) = 19.5$

Thus,
$19.305 - 19.5 < x < 19.695 - 19.5$
$-0.195 < x < 0.195$

It shows that the customer will be overcharged and undercharged by the amount of \$0.195 if the scale will be inaccurate.