1. Firstly write them both out in pence. You would get 100p and 45p.
2. Then put the denominator (100p) on the bottom and the numerator (45p) on top.
3. Now you have 45/100. You can cancel this down by dividing the denominator and numerator by the same number to get the smallest possible denominator when the numerator is still a whole number (e.g 1 or 2, not 1.5).
45/100 = 9/20