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(1) Insufficient. If # of male students = x, then # of female students = 840-x and we can't solve for the value of x.
(2) Assuming # of students in the class = x, # of male students = m, # of female students = x-m, then we can only equate 0.72m+0.8(x-m) = 0.75x to give 0.72m + 0.8x - 0.8m = 0.75x -> we can then solve for x/m. Sufficient.