find the number of ways in which 3 boys and 3 girls (all are of different heights) can be arranged in a line so that boys as well as girls among themselves are in decreasing order of height (from left to right)
permutation and combination
Three boys can be arranged in only one way in decreasing order of height, and so three girls.
Now we can have the group is three boys followed by girls or three girsl followed by boys.
So there seems to be only two different ways, when girls and boys when they arranged among themselves, in decreasing order of height.