Answer: Hello mate!
you have:
8 pairs of pants, 10 shirts and 4 pairs of shoes:
We want to find the total number of possible combinations, where you use a pair of pants, a shirt and a pair of shoes.
These combinations are equal to the product between the number of options for each event (where the events are "selectinc a pair of pants", "selecting a shirt", etc)
For pants you have 8 options, for shirts 10 options and for shoes 4 options, then the total number of combinations are:
8*10*4 = 8*40 = 320
Then there are 320 possible combinations, in a tree diagram you have a leave for every combination, so you have 320 leaves.