<p>For anyone who has the book, on the second practice test in the Blue collegeboard book, on section 6 (math), how do you do the last one? Its the one with the 5 squares asking about probability. The answer is 72 but the book has no explanations. Also, does anyone have any tricks for some of the more tricky probability problems? I know basic permutations and combinations, but questions like this one confuse me...</p>
<p>First focus on the four other squares. They can be arranged in 24 different ways by doing 4! (4<em>3</em>2*1). The shaded block can then be in 3 different places; between the first and second, second and thrid, and thrid and forth no matter what order they are in. So for each one of the 24 arrangements of the other squares, there are three possiblities for the placement of the shaded block, soo 3 * 24 = 72.</p>
<p>thanks a lot!</p>