Question

I have 120 blocks. Each block is one of 2 different materials, 3 different colors, 4 different sizes and 5 different shapes. No two blocks have exactly the same all four properties. I take two blocks at random. What is the probability that these two blocks have exactly two of these four properties the same?

Answer 1

Answer: 0.54

Step-by-step explanatio;

We would have three in common out of 4 characteristics (4 out of 4 is not possible). There are 4 possible ways to share 3 out of 4 characteristics.

Denoting the characteristics as {M,C,Si,Sh} so P(B2={1,1,1,0}|B1={1,1,1,0}) is the probability that box 2 shares its first three characteristics with box 1 given that box one has characteristics

We now have

P(B2 has 3 elements in common with B1)=P(B2={1,1,1,0}|B1={1,1,1,0})xP(B1={1,1,1,0})+P(B2={1,1,0,1}|B1={1,1,0,1})xP(B1={1,1,0,1})+P(B2={1,0,1,1}|B1={1,0,1,1})xP(B1={1,0,1,1})+P(B2={0,1,1,1}|B1={0,1,1,1})xP(B1={0,1,1,1})

P(B2 has 3 elements in common with B1)=(4x5x24+3x4x30+2x3x40+1x2x60)1120x119= 0.084

which is close enough to the value from the simulation code (modified to count the number of 3-ties)

Now, I use the same reasoning to compute:

P(B2 has 2 or more elements in common with B1)=(19x20x6+11x12x10+9x10x12+14x15x8+7x8x15+5x6x20)1120x119= 0.54