How important is it to remember maths formulas Watch
Finding 30% of 20 is the same as doing (30/100) * 20, and then you are adding this on to the original amount, which is 20. If you were to make this look better, this would be 20(1 + (30/100)), which is the same as N0 (N0 meaning the amount you start with) * (1 + r/100), where r is the interest rate, 30%.
You can find the amount at the end of the next year as well by multiplying N0(1-(r/100)) (which is 20 *(1-(30/100))) by multiplying, and this is just a much faster way of doing it.
Have a look at past papers for your exam and see which way they prefer you to use it.