Trying to explain it in the simplest way possible,
The thing we have to keep in mind here is that the elements before 60, i.e. {10,20,40,50} will always be in the same order and the elements after 60, ie {90,80,70} will always be in the same order, but the elements of the two sets can come between each other.
This means that the order 20, 40, 50, 90, 80, 70 is valid, but 10, 20, 90, 30, 40, 80, 70, 50 is also valid.
So imagine we have 7 places to be filled. _ _ _ _ _ _ _
We can chose the 4 elements to be filled in this 7 places in $_{4}^{7}\textrm{C}$ ways. (Not using permutation here as the order of the elements has to be maintained)
Now out of the remaining 3 places there is only choice for the elements of the second set because once the elements of the first set are in their respective places, the second set must appear in only one order: 70,80,90.
So total possible orders: $_{4}^{7}\textrm{C}$ *1 =35