Answer:
b=45/4
Explanation:
This is a special triangle 30-60-90.
In big triangle hypotenuse 15, short leg x=(hypotenuse/2), x=15/2.
In smallest triangle :
1. This is also special triangle 30-60-90
2. x=15/2 hypotenuse
3. "a" is a shortest leg in the smallest triangle,
so a=(15/2)/2=15/4
a+b=15
15/4 +b=15
b=15 -15/4=15*4/4 -15/4 = 45/4
b=45/4