The number of ways they can line up is 3×2×1 = 6. Only one of these is alphabetical order. The probability that they will line up randomly in alphabetical order is 1/6.
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Any of the three can line up first; either of the remaining two can line up second, and the remaining person can line up last. Thus there are 3×2×1 possible orders.
Alex is making a square with 4 corners, brittanie is making a triangle with 3 corners. That's the difference, hope it helped a little, it's not a full answer but maybe it will help.
