2*53/25 = 4.24
4 is not enough to maintain the required ratio, so there must be 5 chaperones, minimum.
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Presumably, the guidelines contain additional information regarding whether the number of chaperones can be odd. Sometimes, two are required so that one can be taking care of a student while the other handles the remaining students (or calls for emergency aid). In that case, the minimum for 53 students may be 6 chaperones.
