lines drawn in a plane Seventeen lines drawn in a plane, with no three concurrent and no two parallel divide the plane into closed regions and open regions. What is the number of closed regions?

I figured out the number of regions is 154 but I am not able to figure out the number of closed regions.
The answer to this question would be 120. As you can see from a chart
number of lines   0 1 2 3 4 5
number of closed  0 0 0 1 3 6  

The formula to find the number of closed should be: n-2(n-1)/2
If we input 17 then, 15*16/2 is equal to 120.
The answer to this question would be 120. As you can see from a chart
number of lines   0 1 2 3 4 5
number of closed  0 0 0 1 3 6  

The formula to find the number of closed should be: n-2(n-1)/2
If we input 17 then, 15*16/2 is equal to 120.