Skip to main content
Back to Permutations & Combinations
JEE Main 2020
Permutations & Combinations
Permutations and Combinations
Easy

Question

Let n > 2 be an integer. Suppose that there are n Metro stations in a city located along a circular path. Each pair of stations is connected by a straight track only. Further, each pair of nearest stations is connected by blue line, whereas all remaining pairs of stations are connected by red line. If the number of red lines is 99 times the number of blue lines, then the value of n is :

Options

Solution

Key Concepts and Formulas

  • Combinations: The number of ways to choose kk items from a set of nn distinct items, where order doesn't matter, is given by (nk)=n!k!(nk)!{n \choose k} = \frac{n!}{k!(n-k)!}. In this case, we use (n2)=n(n1)2{n \choose 2} = \frac{n(n-1)}{2} to find the total number of lines connecting pairs of stations.
  • Circular Permutations: In a circular arrangement of nn objects, each object has two immediate neighbors. Therefore, there are nn connections between nearest neighbors.
  • Problem Translation: Converting the word problem's conditions into a mathematical equation is crucial. In this case, the number of red lines is 99 times the number of blue lines.

Step-by-Step Solution

Step 1: Calculate the Total Number of Possible Lines

  • What and Why: We need to find the total number of lines that can be drawn between any two stations. Since each pair of stations is connected by a line, we use combinations to choose 2 stations out of nn. Order doesn't matter (station A to B is the same line as B to A).
  • Calculation: Let NtotalN_{total} be the total number of lines. Ntotal=(n2)=n(n1)2N_{total} = {n \choose 2} = \frac{n(n-1)}{2}

Step 2: Determine the Number of Blue Lines

  • What and Why: Blue lines connect nearest neighbor stations. In a circular arrangement, each station has two neighbors. Therefore, there are nn such connections.
  • Calculation: Let NblueN_{blue} be the number of blue lines. Nblue=nN_{blue} = n

Step 3: Determine the Number of Red Lines

  • What and Why: Red lines connect all non-nearest neighbor stations. To find the number of red lines, we subtract the number of blue lines from the total number of lines.
  • Calculation: Let NredN_{red} be the number of red lines. Nred=NtotalNblue=n(n1)2nN_{red} = N_{total} - N_{blue} = \frac{n(n-1)}{2} - n Simplifying the expression: Nred=n(n1)2n2=n(n12)2=n(n3)2N_{red} = \frac{n(n-1) - 2n}{2} = \frac{n(n-1-2)}{2} = \frac{n(n-3)}{2}

Step 4: Formulate the Equation from the Given Condition

  • What and Why: The problem states that the number of red lines is 99 times the number of blue lines. We translate this into an equation using the expressions we derived for NredN_{red} and NblueN_{blue}.
  • Equation: Nred=99×NblueN_{red} = 99 \times N_{blue} Substituting the expressions: n(n3)2=99n\frac{n(n-3)}{2} = 99n

Step 5: Solve the Algebraic Equation for nn

  • What and Why: Now we solve the equation for nn. Since n>2n > 2, we know n0n \neq 0, so we can safely divide both sides by nn.
  • Solving: n(n3)2=99n\frac{n(n-3)}{2} = 99n Divide both sides by nn (since n>2n > 2): n32=99\frac{n-3}{2} = 99 Multiply both sides by 2: n3=198n-3 = 198 Add 3 to both sides: n=201n = 201

Common Mistakes & Tips

  • Confusing Total Lines with Red Lines: Remember to subtract the blue lines from the total lines to get the number of red lines.
  • Circular vs. Linear Arrangements: Understand that in a circular arrangement, each element has two neighbors, leading to nn connections.
  • Algebraic Simplification: Be careful with algebraic manipulations to avoid errors.

Summary

We calculated the total number of lines, the number of blue lines, and the number of red lines in terms of nn. Using the given condition that the number of red lines is 99 times the number of blue lines, we set up an equation and solved for nn, finding that n=201n = 201.

Final Answer

The final answer is 201\boxed{201}, which corresponds to option (A).

Practice More Permutations & Combinations Questions

View All Questions