Carcas, can you please show how you applied the "ratio strategy"?
because you have (make the things simple) FOR 3x + 7x = 100 so your UM is 7x (70 of bunuel): with 7x we have not real number but 1) provide us with.
IF 7x is the total number of students AND 40 not respond so 60 respond. IF you have 7x = 60 (real number) you CAN calculate the number of student (remember: we are talking of integers, so even you have 14.5 is not possible, is a constraint not written but you have to take in account for). However, it depens a lot from the context.BE careful
: this is totally an inference and intuitive approach, that come up after several time of study and that you develope instinctively. Similar native speakers have about SC
KUDOS is the good manner to help the entire community.