a question about discount "70%off" , american english and british english seems interpret it in opposite way am i right

I'm not aware of any difference- could you tell us how you think British and American english would respectively interpret it?

If something was originally $100 and there is a 70% off discount, it’s $30 both in the US and the UK.

In the US, this means that you multiply the price by 0.3 (or, strictly speaking, that you subtract 0.7 times the price from the price). I’d be a bit surprised if this means something different in British English.

Any chance it's the difference between "percent off" and "percent of"?

England, 70% off means something that cost £10 will cost £3. I'm fairly sure that's universal. https://i2-prod.manchestereveningnews.co.uk/incoming/article26368898.ece/ALTERNATES/s810/0_IMG-6003.jpg That's from a sale in Boots the chemist (= pharmacist), in Manchester (England).

As an American, we typically say what percent of the price is removed as opposed to what percent of the price you're expected to pay. I couldn't tell you why it's that way, but it just is. As far as I'm aware, it's the same way in Britain, but I could be wrong.

Adding onto what others have said: "70% off" is the same as "reduced to 30%".

As a Brit I would assume that something that was £100 now costs £30. Is it any different in the US?

In English, the preposition '*off*' is very important. You are overlooking how important prepositions are when talking about values. In this context, '*off*' means 'removed from'. So '*70% off*' means that 70% has been removed from the original price. There is no other interpretation of this. If I said "*it's 70% the original price*" or "*it's 70% of the original price*" or "*the price is now 70% (of) what it was*", then this would mean '*30%*' off. The only phrase that *might* be ambiguous (confusing) is '*discount*'. This means '*money off*' and should always be understood this way. However, you could also say "*there's a discount. It's 70%*' which would be confusing; is it the price that's now 70% or the discount that's 70%?

In all English dialects, 70% off means 70% is taken _off_ of the initial price. So something that was $100 would be $30.

Yes. In American English, it means that the price of the item has been reduced by 70%. For example, if an item originally costs $100, it would now cost $30. In British English, it can either mean that the price of the item has been reduced by 70%, as in American English. Or, it can also mean that the price of the item has been reduced to 70% of its original price. For example, if an item originally costs $100, it would now cost $70.