Interview Q&A / Aptitude

1. What is the difference between permutations and combinations, and how do you know which to use?

   Use permutations when order matters, like choosing a captain and vice-captain, and combinations when order does not matter, like choosing a committee. nPr = n! / (n - r)! and nCr = n! / (r! x (n - r)!). Each combination of r items can be arranged in r! ways, so nPr = nCr x r!.

   AdvancedIn-depth4 min read
2. How many 4-digit numbers can be formed using the digits 0, 1, 2, 3, 4 and 5 without repetition?

   300. The first digit cannot be 0, so it has 5 choices. The remaining three places use the other 5 digits: 5 x 4 x 3. Total = 5 x 5 x 4 x 3 = 300.

   BeginnerQuick answer1 min read