I did the Full quant section for practice and found Q 18 a bit boggling, saw the video explanation were a student answered how she solved it but it didn’t really understand it.

hi, well I am sure this method is confusing a bit i would suggest u to go for alternative method which is choosing numbers.

where they said BM=3CM so lets assume cm =3 and so BM will be 9 and since its similar triangle we can see the change of difference from one side and use it for other side .By doing so we can find NC is 2.5 so the total length of AC will be 7.5+2.5=10.

By Side Splitter Theorem, we can say " if a line is parallel to a side of a triangle and it intersects the other two sides, it divides those sides proportionally. "

Thanks!! your post made me realize that the “CM” in BM = 3CM was not talking about centimeters. I know they usually don’t talk about units in this types of questions but i got confused.