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The problem is presented VERY INACCURATELY in your post.
The correct formulation is THIS:
There are 756 male students at a college. The ratio of male students to female students is 7:3.
How many female students are there at the college ?
Try to write your posts correctly.
It is very difficult to decipher posts written inaccurately.
And it is VERY UNPLEASANT work.
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Be aware ! : The answer "354" by @josgarithmetic is W R O N G (!)
The correct answer is 324, given by Alan.