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15 Jun 2020, 16:28
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Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
95%
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Question Stats:
45% (02:44) correct
55%
(02:59)
wrong
based on 77
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Average marks of Sandeep are decreased by 2 when he exclude 40 marks of social science and include 28 and 30 marks of physics and chemistry respectively. If he again add 54 marks of computer science the average becomes equal to original average. Find the number of subjects originally.
(a) 8
(b) 9
(c) 10
(d) 12
(e) 15
(a) 8
(b) 9
(c) 10
(d) 12
(e) 15
yashikaaggarwal
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Joined: 19 Jan 2020
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16 Jun 2020, 05:48
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Let Average Be A
No. of subjects be N
Total MARKS in subjects = T
therefore, T = AN
Average marks of Sandeep are decreased by 2 when he exclude 40 marks of social science and include 28 and 30 marks of physics and chemistry respectively.
A-2 = AN-40+28+30/N-1+2
A-2 = AN+18/N+1
(A-2)(N+1) = AN+18
AN-2N+A-2= AN+18
A-2N = 20 ------------(1)
If he again add 54 marks of computer science the average becomes equal to original average.
A = AN+18+54/N+2
A = AN+72/N+2
(A)(N+2) = AN+72
AN+2A= AN+72
2A = 72 => A = 36 ------------(2)
Putting Value from equation 2 in equation 1
A-2N = 20
36-2N = 20
16 = 2N
N = 8
Answer is 8
No. of subjects be N
Total MARKS in subjects = T
therefore, T = AN
Average marks of Sandeep are decreased by 2 when he exclude 40 marks of social science and include 28 and 30 marks of physics and chemistry respectively.
A-2 = AN-40+28+30/N-1+2
A-2 = AN+18/N+1
(A-2)(N+1) = AN+18
AN-2N+A-2= AN+18
A-2N = 20 ------------(1)
If he again add 54 marks of computer science the average becomes equal to original average.
A = AN+18+54/N+2
A = AN+72/N+2
(A)(N+2) = AN+72
AN+2A= AN+72
2A = 72 => A = 36 ------------(2)
Putting Value from equation 2 in equation 1
A-2N = 20
36-2N = 20
16 = 2N
N = 8
Answer is 8
13 Jan 2022, 21:57
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I think the conventional substitution method is time consuming. Apart from the usual plug-in method, try prime factorisation for 28,30,40,54==>> 2^3.3^3.5^1.7^1.
now sum the powers==> 3+3+1+1=8.
Try this only when you're out of time and in need of an educated guess.
now sum the powers==> 3+3+1+1=8.
Try this only when you're out of time and in need of an educated guess.
