A machine is set to fill 500 g packets, and a packet clears inspection only if its weight is within 3% of 500 g. A packet on the belt weighs 518 g. By how many grams does it exceed the heaviest weight allowed?

The allowance is 3% of 500 = 15 g, so a packet may weigh anything from 485 g to 515 g. At 518 g it is over the top of that band by 518 - 515 = 3 g. 18 g measures the excess over the 500 g setting and forgets that a 15 g allowance exists. 15 g quotes the allowance itself. '0 g - the packet passes' is the trap of comparing 518 with 500 + 18 or of assuming 3% is generous; the band closes at 515 g.