A+C=B/2

B+B/2=π

B=2π/3

The area of triangle ABC is:

1/2 * AC * sinB = 1/2 * AC * sin 2π/3 =√3/4 * AC = 15，

ac=20√3，

Cosine theorem: Cosb = (A 2+C 2-B 2)/2ac,

So A2+C2-B2 = 2ac * COSB = 2 * 20 √ 3 * COS2π/3 =-20 √ 3,

a^2+c^2+2ac-b^2=2*20√3-20√3=20√3，

(a+c)^2-b^2=20√3，

(a+c+b)(a+c-b)=20√3

And a+b+c=30,

So a+c-b=2√3/3,

2(a+c)=30+2√3/3

a+c= 15+√3/3

ac=20√3，

Solve the equation and find out a, c,

While a+c= 15+√3/3 and a+b+c=30, it is easy to find b,

I told you the method, so you can do the math yourself.

2a^2