Did they also teach it backwards, i.e. if the total/gross was £115, then the VAT was £15 i.e. 15% on the net £100, and not 15% of the gross £115 which is £17.25.
That is the biggest issue I see as an accountant, that even highly educated people like doctors, IT consultants, architects, etc., still do their book-keeping and VAT returns by wrongly multiplying the total paid (gross) by 20% VAT to find the VAT element of a payment, rather than doing it as 100/120.
Same with people like tradesmen who give a customer a quote for £200, VAT inclusive, and then write an invoice for £160 plus £40 VAT, i.e. just starting backwards with the total of £200, then saying the VAT is 20% of that, i.e. £40, and working backwards to a net of £160!
The sheer number of people who don't fully understand the difference between net and gross is unbelievable. People really shouldn't be leaving school and making that kind of really basic mistake, after 10+ years of compulsory Maths/numeracy education, even more so if VAT is specifically part of the Maths/PHSE curriculum.