A factory's records show that 3 items in every 250 produced are defective. In a consignment of 4,000 items, how many defective items would you expect?

The consignment holds 4000 ÷ 250 = 16 groups of 250 items, and each group is expected to carry 3 defectives, so the expected number is $16\times 3=48$. The same comes from the rate: $(3/250)\times 4000=48$. Reading '3 in every 250' as 3 percent gives 120, more than twice too many, since 3 in 250 is only 1.2 percent. The value 16 is the number of groups with the multiplication left undone, and 24 uses 500 as the base instead of 250.