The given triangle is a right angle triangle with 90 degree at angle A :
where AC = 20, CB = 25
Apply the pythagoras theorem for the side BA
Pythagoras Theorem : In a right-angled triangle, the square of the hypotenuse side is equal to the sum of squares of the other two sides
Hypotenuse² = Perpendicular² + Base²
In the given triangle BAC :
Hypotenuse = CB, Perpendicular = AC, Base = BA
substitute the value :
Hypotenuse² = Perpendicular² + Base²
CB² = AC² + BA²
25² = 20² + BA²
625 = 400 + BA²
BA² = 625 - 400
BA² = 225
BA = 15
The third side is 15
Answer : 15