Grades 9-10 Video Solutions 2025-2025_9-10

Video Transcription

Video Summary

The problem involves determining the probability that brick 4 is the third brick removed from a stack of five bricks, where Peter can only remove a brick if no other bricks are on top of it. Initially, Peter can remove either brick 1 or 2, with no restrictions. For brick 4 to be third, bricks 1 and 2 must be removed first. After removing the first brick (either 1 or 2), there's a 50% chance of picking correctly for the second move. After removing bricks 1 and 2, there's a 1 in 3 chance of picking brick 4 as the third. Therefore, the probability is \( \frac{1}{6} \).

Keywords

probability

brick removal

stack of bricks

combinatorics

probability calculation