1. Can you have MORE than 100%?

Yes. 200% of something is double what you started with.
150% is half as much more than you had to start with.

1000% is ten times as much as you had!

2. What is the quickest way to decrease, say, 120 by 5%?

You could find 5% of 120 and then subtract it from 120 but... the QUICKEST way is to multiply 120 by 0.95.

3. When you do division of fractions, which ones do you turn upside down ?

All except the first one. And remember that the operation is changed from division to multiplication.

4. Can you cancel two numbers both on the top lines of fractions?

No, cancelling HAS to be between a number the top line and any number on the bottom line. (This can be done diagonally.)

5. When adding fractions, is it best to change mixed numbers to improper fractions?

Yes, unless the calculation is VERY easy, mixed numbers should be changed to improper fractions for adding, subtracting, multiplying and dividing.