Hi, can someone help with this? Have watched both factor and multiple videos from prep swift but unsure how to solve this. I understand that one of the criteria to choose the answer is that the 990 which is a multiple ought to be divisible by the answers - therefore, only 10 and 11 are valid but why 11 instead of 10?

990 has 11 has a factor, so you must have at least 11, which 10! does not have.

Got it. So the easiest way is to do prime factorisation and assess which factor is missing. In this case, give the options, only 11 fits the bill.