'Sums' is what we tend to call it in primary school/first school/infants school, before we learn to call it 'maths' (math en-US) or 'arithmetic'. Although people may still say 'do the sums' and older generations especially may refer to it in that way. So it's not just addition in this case. However, 'to sum' something does mean 'addition', e.g. 'I'll sum it up' meaning to 'total' up amounts. Separately, 'to sum up' can be used to mean 'in summary' / 'to summarise' (summarize en-US).
Another common expression, much favoured by politicians, is, 'the sums just don't add up', although in this case 'sums' is really a synonym for 'amount(s)'. 'Large sums of money' would be another example.
Incidentally, 'five and four sums to nine' would be a valid expression but is not typical.
Sorry, I suspect I haven't explained that terribly well. So I hope I haven't confused you more. Hopefully you get the idea?
This is the first course on Duo which uses British English as a basis that I have seen, although a few words and phrases have creeped in on other languages. Of course it makes sense since Great Britain certainly must have the majority of Welsh speakers. I did hear it as a child from time to time in and around upstate New York but I doubt I would nowadays. I have heard that there is an area of Argentina where they speak Welsh.
I can't speak for the exact meaning of "Symiau", but I think it's more general than "addition". I think "math" is the closest. As kdb119 says, "sums" are what young children do in school, and it can be any sort of mathematical operation. In that sense "10 minus 5" can be "a sum", or "6 divided by 2".
Hmm... Yes; in the sense of 'summing up', perhaps - except that 'sums' which is the given translation, would include subtraction. In the wider sense of 'maths'; not really. It depends upon the accepted sense of 'Symiau', which I'm not sure about.
Ap Geiriaduron and GPC suggest the singular 'Sum'/'Swm' - which suggests only summing up/totalling. In which case you would seem to be right.
Perhaps a native Welsh speaker can clarify what is generally understood for 'Symiau'?
Yes, but as you implied earlier, kdb, "sums" is the colloquial term for "arithmetic" at the primary school level (throughout GB; not just in Wales). It covers addition, subtraction, multiplication, and division. (So "addition" doesn't really cover it.) In my experience, "symiau" maps exactly onto "sums" in this (British) sense.
Of course, whatever corresponds to this definition in American, Australian, etc. English should be accepted as a valid answer too!
Possibly because 'sums' is a much less formal and mostly childhood term for 'arithmetic’ and 'maths' is a less formal term (abbreviation) for 'mathematics'.
On the face of it, once 'math(s)' becomes an acceptable answer, it does seem a little odd/unreasonable that 'mathematics' is not, but strictly speaking mathematics and arithmatic are not at all the same thing.
I suspect 'maths' has been included merely to placate non-Brits who don't understand our usage of the word 'sums' and thus expect 'math' to be acceptable. As is apparent from other discussion topics, Americans in particular do get very worked-up and vociferous when they don't immediately understand British (or any non-US) usage. If you allow 'math' you have to also allow 'maths' otherwise Brits will complain about pandering to US-bias. :-) That said, I'm not sure if 'arithmetic' is actually accepted as a correct answer. From what native-speakers have said above, I suspect that really the only truly correct answer should be 'sums'.