The sample space is Ω = {1234, 1243, 1324, 1342,..., 4321}. The best way to count its elements is by noting that for each of the 6 outcomes of the threeenvelope experiment we can put a fourth envelope...

The sample space is Ω = {1234, 1243, 1324, 1342,..., 4321}. The best way to count its elements is by noting that for each of the 6 outcomes of the threeenvelope experiment we can put a fourth envelope in any of 4 positions. Hence Ω has 4 · 6 = 24 elements.