QUESTION

CSAT

Easy

Maths

Prelims 2023

In an examination, the maximum marks for each of the four papers namely P, Q, R and S are 100. Marks scored by the students are in integers. A student can score 99% in n different ways. What is the value of n? Let pp, qq, rr, and ss be the marks scored in papers P, Q, R, and S respectively. Since the maximum marks for each paper is 100, we have 0p,q,r,s1000 \le p, q, r, s \le 100. The total marks is p+q+r+sp+q+r+s. The maximum possible total marks is 4×100=4004 \times 100 = 400. 99% of the maximum marks is 0.99×400=3960.99 \times 400 = 396. We need to find the number of integer solutions to the equation p+q+r+s=396p+q+r+s = 396, subject to the constraint 0p,q,r,s1000 \le p, q, r, s \le 100.

Without the upper bound restriction, the number of non-negative integer solutions is given by (396+4141)=(3993)\binom{396+4-1}{4-1} = \binom{399}{3}. However, we have the constraint p,q,r,s100p, q, r, s \le 100. Let p=100pp' = 100 - p, q=100qq' = 100 - q, r=100rr' = 100 - r, s=100ss' = 100 - s. Then 0p,q,r,s1000 \le p', q', r', s' \le 100. p+q+r+s=396p+q+r+s = 396 (100p)+(100q)+(100r)+(100s)=396(100-p')+(100-q')+(100-r')+(100-s') = 396 400(p+q+r+s)=396400 - (p'+q'+r'+s') = 396 p+q+r+s=400396=4p'+q'+r'+s' = 400-396 = 4 The number of non-negative integer solutions to p+q+r+s=4p'+q'+r'+s'=4 is (4+4141)=(73)=7×6×53×2×1=35\binom{4+4-1}{4-1} = \binom{7}{3} = \frac{7 \times 6 \times 5}{3 \times 2 \times 1} = 35. Thus, n=35n = 35.

Final Answer: The final answer is 35\boxed{35}

Select an option to attempt

Explanation

The maximum marks = 100+100+100+100=400100 + 100 + 100 + 100 = 400

The marks scored by the student = 99%99\% of 400=396400 = 396

So, basically he has lost 4 marks in total. He can lose these 4 marks in the following ways:

  • (4,0,0,0)(4, 0, 0, 0): Scored 100 in three papers, and scored 96 is one paper. This can be done in 4!/3!=44!/3! = 4 possible ways.

  • (3,1,0,0)(3, 1, 0, 0): Scored 100 in two papers, 99 in one and 97 in one. This can be done in 4!/2!=124!/2! = 12 possible ways.

  • (2,1,1,0)(2, 1, 1, 0): Scored 100 in one paper, 99 in two papers, and 98 in one. This can be done in 4!/2!=124!/2! = 12 possible ways.

  • (2,2,0,0)(2, 2, 0, 0): Scored 100 in two papers, and 98 in other two. This can be done in 4!/(2!2!)=64!/(2! 2!) = 6 possible ways.

  • (1,1,1,1)(1, 1, 1, 1): Scored 99 in all the four papers. This can be done only in 1 possible way.

So, n=4+12+12+6+1=35n = 4 + 12 + 12 + 6 + 1 = 35

Hence, option (D) is correct.

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