How many combination can be made with 5 shirt, 3 skirts & 7 sandals?

There are 105 ways.

Let's think about it in combinatoric form.
We have 5 ways to choose 1 shirt.
So the number of ways to choose a shirt is 5C1 = 5

We also have 3 ways to choose 1 skirt.
Number of ways is: 3C1 = 3

We have 7 ways to choose 1 sandal.
Number of ways is: 7C1 = 7

We want them simultaneously (ie we want to have different shirt, skirt and sandal combos), so we multiply the ways to form 105 different ways.