The mean of set of 30 observations is 75. If each observation is multiplied by a non-zero number $\lambda $ and then each of them is decreased by 25, their mean remains the same. Then $\lambda $ is equal to :
${1 \over 3}$
${2 \over 3}$
${4 \over 3}$
${10 \over 3}$
Solution
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