Unga wa mahindi is not used?
Why is "Ninyi mnataka sembe kilo ngapi?" not accepted?
This looks like an incorrect translation?
Swahili can sometimes have various types of sentence structure. It can also be translated to 'mnataka kilo ngapi za sembe?', which is closer in structure to the original sentence
"Mnataka kilo ngapi ya sembe" should be accepted.