Basic math skills lacking

woolworths is so very quick at advertising the less 20%. I go to the counter after selecting my two pair of shoes the less 20% didn't kick in automatically. I then ended up wasting 40 mins of my life watching the manager do Maths trying to figure out how to deduct 20% off two shoes. What a joke! Four people tried to help but unfortunately none of them could figure out how to calculate the 20%. Shocking and ridiculous