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newland2010, your comprehension was wrong! 30 swam in deep and shallow ends (BOTH). The question clearly asks for the number of people who swam in deep end ONLY and shallow end ONLY. my answer was...

let s = number of people who swam in the shallow end only
let d = number of people who swam in the deep end only

since 30 swam in both deep and shallow ends then we eliminate them from the total....