Why is "I like mathematics" an incorrect translation of "Dw i'n hoffi symiau" while "I like maths" is considered correct"?
On the one hand, if it accepts "maths", you could argue that it ought to accept "mathematics". On the other hand, mathematics is not the same as "sums". Sums are more properly called "arithmetic" because they only involve the basics of adding, subtracting, multiplying and dividing. Basically what you would do in the early years at primary school.