+125 the weights of grapefruits in a bin are normally distributed with a mean of 261 g and a standard deviation of 9.4 g
approximately 20% of the grapefruits weigh less than which amount?
293 g
247g
253g
261g
+6250 \(\text{This one is the reverse of the others}\\ \text{Let }w \text{ be the weight you're after in this problem}\\ 0.2 = \Phi\left(\dfrac{261-w}{9.4}\right)\)
\(\Phi^{-1}(0.2) = \dfrac{261-w}{9.4}\\ \text{So use your table or software or whatever to look up the argument, }x,\\ \text{to }\Phi(x) \text{ that corresponds to }\Phi(x) = 0.2\\ \text{Once you've got that value simple algebra will solve for }w\)
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