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Explanation:
There are 3 colors of sweaters {red, green, purple} and 2 colors of skirts {tan, black}
So far there are 3*2 = 6 different outfit combinations. Imagine a table with 3 rows representing the sweater colors and 2 columns representing the skirt colors. There would be 6 cells in the table representing each possible combination. One cell would have (red sweater, tan skirt) for instance.
For each of those 6 combinations, we also have 2 shoe colors. So overall we have 6*2 = 12 outfit combinations
The tree diagram is shown below. Note how there are 3*2*2 = 12 different paths to follow.