1. If someone scores a mark of, say, 8 out of 10 in a test, how is it changed to a percentage?

8 out of 10 is the same as the fraction eight tenths
For a percentage, the bottom number has to be 100.
As 8/10 = 80/100, the percentage would be 80%.

2. Can I use my calculator to work out percentages?

Some calculators have a % button but it must be used carefully.

It is much better to UNDERSTAND percentages and then use your calculator to do the multiplying and dividing. This usually involves multiplying or dividing by 100.

3. Can you have MORE than 100%?

Yes.

101% is 1% more than you had before.
200% of something is 100% more than you had, or double what you started with.
150% is half as much again.