The second one which you would solve 5/20and -3/45
This is just some pythagorean theorem, or a^2 + b^2 = c^2. We have a and b, which is 9 and 12, and we need to find c. To find c, we need to first square those, giving u 81 and 144. Now, we need to add them, giving us 225, meaning c^2 = 225. However, we need to find c on its own, so we have to find the square root of 225. That would be 15.
Your limting the amount of sandbags you have if you take away and when your adding your adding more sandbags.