### Coloured Run of Cards

Source: http://www.quantnet.com/forum/threads/colored-runs-of-cards.6508/ Problem: There are 26 black(B) and 26 red(R) cards in a standard deck. A run is a maximum block of consecutive cards of the same color. For example, a sequence RRRRBBBRBRB of only 11 cards has 6 runs; namely, RRRR, BBB, R, B, R, B. Find the expected number of runs in a shuffled deck of cards. Update (29-05-2011): Solution: Posted by Siddhant Agarwal (EE IITB 2011 Alumnus) in comments! Select the text between * to see the solution. * Let us have n red cards and n black cards. Consider any sequence X_1,X_2..X_2n. Now define Y_1 = 1, Y_i = 1 if X_i and X_(i-1) are of diff colours Y_i = 0 otherwise. Clearly we have to find E[Y_1 + ..+Y_2n] E[Y_1] = 1 by def. E[Y_i] = E[Y_j] for all i,j>=2 by symmetry. Also E[Y_i] = $2*(\binom{2n-2}{n-1})/($\binom{2n}{n}) so ans = 1+(2n-1)*E[Y_i] = n+1 *