1 fathom = 6 feet
2 1/2 fathoms = x feet
2 1/2 fathoms • 6 feet = x feet
x = 15
2 1/2 fathoms = 15 feet
So the sum of 17 and a number x means to add them on one side of the equals sign. The other side would be your 20.
17 + x = 20
Since 1/2 is one half, the rise must be half of the run, meaning that the rise would be 25.
