PPC Cement have a new radio advert, in which a builder goes in to a store to buy 100 bags of cement but is only given 85. When he asks why, he's told that "PPC Cement goes 15%" further... that may be true, but did you spot the mathematical error that's going to leave builder unable to complete his job?

It's true that 15% of 100 bags is 15 (100 / 100 x 15 = 15). It's also true that 100 bags minus 15 bags(%) = 85 bags.

What's NOT true is that 85 bags + 15% (of 85 bags) = 100... because 85 / 100 x 15 = 12.75

The builder is going to be 2.25 bags short worth of cement short, because 85 bags + 12.75 bags of "15% extra) is only 97.75 bags worth.

I would not want that buider, store owner, or PPC Cement involved in the next building I move in to!