It is apparent that we are talking about a right angled triangle, whose two sides #A=20# and #C=25#. In a right angled triangle, according to Pythagoras Theorem, square on the largest side, which is a hypotenuse, is equal to sum of the squares on other two sides.

Here we have two possibilities.

**One** - #C=25# is the hypotenuse and the largest side. As it's square is #25^2=625# and square of #A# is #20^2=400#, the square of third side would be #625-400=225# and

third side, say #B=sqrt225=15#.

**Two** - #B#, the third side is the hypotenuse and the largest side. As sum of the square of other two sides #A# and #C# is #20^2=400# and #25^2=625#, the square of third side #B# would be #625+400=1025# and

third side, #B=sqrt1025~=32.02#.