How many ways can you purchase 3 CDs if there are 3 to choose from, 4 cassettes if there are 7 to choose from, and 2 DVDs if there are 4 to choose from?

3C3 X 7C4 X 4C2 = 210

Here's a rule of thumb that works for these probability questions.

Let say there are 5 objects and you can 3 of them. Then the total number of ways are:

5x4x3=60

This is because at the beginning there are 5 objects to choose from, after one is chosen, there's only 4 left. After another one is chosen, there is only 3 left. That's when you are done with your 3 turns to choose.

I am a bit busy and I will tell you later