Question

out of 6 letters and 6 adressed envelopes how many ways can exactly two letters be placed into their correspomding envelopes​

Answer 1

Step-by-step explanation:

135

2 letters can go to their corresponding envelopes in 6C2 or 15 ways.

The remaining 4 letters should go to wrong envelopes in 9 ways:

You can use my derangement formula, Sn = (Sn-1 + Sn-2)*(n-1), where S1=0, S2=1, S3=2.

S4 = (S3+S2)*(4–1).

S4 = 3³= 92

Therefore, total arrangements equal 9*15 = 135.