Two of the three sides of a triangle are 20 and 15. which of the following numbers is not a possible perimeter of the triangle?

Perimeter of the triangle is 60.

A perimeter of a triangle can be found using the following:
1) measurement of 3 sides are given:  
P = a + b + c
2) 2 sides of a right angle triangle is given:
a² + b² = c² ; P = a + b + c
3) 2 sides of a triangle is given with the measurement of an angle. 
c² = a² + b² - 2ab(cos(C)).

Since, only the measurement of 2 sides is given, we can assume that the triangle is a right-angled triangle.
c² = a² + b²
c² = 20² + 15²
c² = 400 + 225
c² = 625
c  = √625
c  = 25

P = a + b + c
P = 20 + 15 + 25
P = 60