My Q is not come in the box so i put it in the following box.

The answer is n ( n-1) /2

first case:

From the nth level

n n-1

nn-2

n 1   total lines from nth level to 1st level =  (n-1) lines

second case is for n-1 level

n-1n-2

n-1n-3......so on

Total lines = n-3 lines

like this if we go on continuing , the number of lines comes as (n-1) +( n-2) + (n-3) +.....2+1 = n(n-1)/2