Question
The average weight of a class of 25 student is 60 KG. If
X is student joined the class,  than the average weight of whole class increases by (2X – 7)Kg. Find the value of X if the weight of whole class increases by 390 KG.Solution
Sum of weights of 25 student = 25*60 = 1500 kg Sum of the weights of (25+x)(60+2x-7) = {(25+x)(53+2x)} kg As per the question (25+x)(53+2x)-1500 = 390 (25+x)(53+2x) = 1890 2x² + 103x – 565 = 0 X= 5 or (-113/2) Student since the number of student cannot be negative. So our answer will be 5.
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