There are two sizes of notepads and four different colors for each notepad.

The store sells two categories of notepads:

I same size and same color
II same size and all different colors.

Let's consider I first. There are four different colors. So, we can have all blue or all green etc. But there are two different sizes. So we can have all small/blue or all large/blue or all small/green or all large green etc. So, in I, there are 4*2 or 8 different types of notepads.

For II, we have four colors from which we are selecting any three. So, there are 4C3 = 4C1 = 4 ways we can have three different colors. Again, there are two different sizes, so here we also have 4*2 = 8 different types of notepads.

So, all told there are 8 + 8 = 16 types of notepads.

Thank you so much. I was multiplying the result of I & II. Now i understand it.

harmeet wrote:Thank you so much. I was multiplying the result of I & II. Now i understand it.

No worries. In combinatorics/probability, when you have "or" you add and when you have "and" you multiply. Here, we can have a notepad package in I OR a notepad package in II.