Lets say you have 5 apples, but the you give away 3 of them.
To work how many you have left you take away.
5-3 = 2
Lets apply this with fractions now.
You have 9/7 yard, but then you give away 7/20 yard.
Now you take them away:
9/7 minus 7/20 = 131/140
Hope this helps :)
