159381

My answer was 21.

I got the answer by using guess check and improve and logical thinking.

First I tried to figure out the circle, I chose a number which was less than 5 (I chose 4).

Next, I tried to figure out the square (20-4x3=8), which means that square is 8. Then, I tried to figure out the triangle (30-8x3=6).

After that, to figure out the hexagon, I did (30-8x2)÷2. At last, I used the amounts of the shapes to figure out the answer.