Can you name some math terms starting with the letter "o" and give a short description of them?

Obtuse angle

Operation

Order of operation

"Order of operations" - the order in which you must simplify an equation 
"Origin" - center of the coordinate plane at (0,0)
"Outlier" - A number in a set of numbers that is way different than the rest (example: 1, 2, 3, 4, 5, 27. 27 is the outlier)
"octagon" - an 8 sided polygon 

It would be 15 m how i got the answer: (20x20) + b^2 = (25x25)400 + b^2 = 625625-400=225square that and get 15hope this : )