ugo_castelo

The figure shown represents a board with 4 rows of pegs, and at the bottom of the board are 4 cells numbered 1 to 4. Whenever the ball shown passes through the opening between two adjacent pegs in the same row, it will hit the peg directly beneath the opening. The ball then has the probability 1/2 of passing through the opening immediately to the left of that peg and probability 1/2 of passing through the opening immediately to the right. What is the probability that when the ball passes through the first two pegs at the top it will end in Cell 2?
A. 1/16
B. 1/8
C. 1/4
D. 3/8
E. 1/2
The sieve is symmetric about the center.
Without doing much math can guesstimate with reasonable chance of success.
Slots 2&3 should have equal probability, slots 1&4 equal but less than the others.
2&3 could be weighted 2 and the others 1, so slot 2 could have probability of 2/6, but this doesn't appear as an answer.
2&3 could be weighted 3 and the others 1, so slot 2 could be 3/8.
A. can be eliminated because 2&3 together would add to 1/8, meaning the other two are 7/8, not possible given that a casual examination shows fewer ways to reach these slots.
B. can be eliminated for the same reason.
C. can be eliminated because it implies equal probability for all slots, not possible.
E. can be eliminated because it implies 0 probability for slots 1&4.
Posted from my mobile device