Compute the area of a triangle with sides. 15 × 15 × 18

You can find the area using Heron's formula.
A=√(s(s-a)(s-b)(s-c)),
where a,b,c are sides of the triangle,
and s=(a+b+c)/2 - half perimeter of a triangle.
a=15,
b=15
c=18
s=(15+15+18)/2=48/2=24
s=24
s-a=24-15=9
s-b=24-15=9
s-c=24-18=6

A=√(24*9*9*6)=√(4*6*6*9*9) = 2*6*9=108
A=108

8doors on the second floor

The first option would be correct if it were one half and not one