12: 5 and 20 aren’t the same numbers meaning you have to make 5= as 20.
You will have to multiply 5 x 4 meaning you would have to multiply 4 to the four on top as well. You can leave 20 alone because that’s the number you are trying to reach. 5 x 4 = 20 and 4 x 4 is 16 so 9/20x 16/20 and 20 is already staying the same so all you need to do is multiply 9 and 16 which is 144. I hoped this helped and an example.
For 13 and 14 since you would have to multiply 7 times 21 and 10 times 21 then that would be multiplied by 20/21 but first you would need to multiply 21 times 10 and 20 times 10 then multiply your answers. Hope this helped:))