Average

5.Of Five numbers, whose average is 60, the first is one-fourth of the sum of the last three. The first number is : A)36 B)37 C)39 D)40 Answer : Option D Explanation: Let the first number be x, Then, sum of the four numbers = x + 5x = 6x. so, 6x/4 = 60 or x = (60 * 4) / 6 = 40. 6.The average price of three items of furniture is Rs. 30000. If their prices are in the ratio 3:5:7, the price of the cheapest item is : A)18000 B)20000 C)16000 D)17000 Answer : Option A Explanation: Let their prices be 3x, 5x and 7x. Then, 3x + 5x + 7x = (30000 * 3) or x = 6000. Cost of cheapest item = 3x = Rs. 18000. 7. If the mean of a, b, c is M and ab + bc + ca = 0, then the mean of is : A)16 B)14 C)15 D)18 Answer : Option C Explanation: We have : ( a + b + c) / 3 = M or (a + b + c) = 3M. Now (a+b+c)2=3M2 =>a2+b2+c2+2(ab+bc+ca)=9M2 =>a2+b2+c2=9M2[ab+bc+ca=0] Required mean =(a2+b2+c2)/3=92/3=3M2 8.David obtained 76, 65, 82, 67 and 85 marks (out of 100) in English, Mathematics,Physics, Chemistry and Biology What are his average marks ? A)75 B)82 C)65 D)68 Answer : Option A Explanation: Average = (76 + 65 + 82 + 67 + 85 )/ 5 = 375/5 = 75.

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