The sequences 10, 20,40, 50 and 90, 80, 70 can then be shuffled together, as long as their sub-sequences keep intact. Thus we can have 10, 20, 40, 50, 90, 80, 70, but also 10, 20, 90, 30, 40, 80, 70, 50.
but 10, 20 ,80,50,90,40,70 its not the possible sequencm when you make BST for it will be ambiguous coz of 90.
In the final 7 positions I have to choose 3 positions for the larger numbers (and the remaining 4 for the smaller numbers). I can choose these in 7C3=35.(73) ways