drewissimo33 is correct in his comments above. I'm from Canada and I'm equally confused by this strange "Division sums". For us, "sums" ONLY ever means the result of addition (or, possibly, subtraction if we're talking about adding negative numbers). It definitely seems like the word "sums" is also equivalent to the word "problems" or "questions" in Wales?
It is really an informal way of saying problems or questions relating to division.
For older children in schools both 'problemau rhannu' and 'cwestiynau rhannu' are used to describe 'division problems' and 'division questions'
symiau rhannu is understood as the same as both.
symiau on its own (sums) is an informal way of talking about arithmetic
It is easily understood by pupils in Welsh medium schools, while the more formal rhifyddeg is hard for pupils to understand because it's not part of everyday speech.
Now children, we're going to do some maths, some math (in America), some mathematics (quite formal to children), some maths problems, some arithmetic, some addition and subtraction, some adding and subtracting (young children, non-technical), some multiplication and division, some multiplying and dividing (young children, non-technical). Grab out your books. Maths is fun! (I agree, the only meaning of "sum" or "sums" is addition.)
'Sums' (plural) can mean any simple arithmetic - so we might do multiplication sums, subtraction sums, division sums, addition sums, even though the end result of an addition is 'the sum' (singular), of a multiplication it is the 'product' and for a division, the 'quotient'. Can't remember any specific term for the result of a subtraction! Children in days gone by were often told 'be quiet and get on with your sums'.