[Zer0]

" I've just seen a TV advert for a detergent that requires "four times less scrubbing" to remove grease. "

I felt They were making an Attempt to boost Sales using Scientific mathemagikz!

Anyways...
Goods Delivered
$500
' Four times Less ' d.r.a.
$125
Total due
$375

The point where i see Their logic break apart is...
If the minimum amount of Scrubs required to clean a surface A___B which has a fixed surface area is Just One Scrub!
(forward & return)

2/4 = 0.5

ps - an invoice/bill is starting off of a process, a receipt is proof of final commencement.
There is Always a Scope for Error Correction in Economics!