A photographer will arrange 6 people of 6 different heights for
photograph by placing them in two rows of thress so that each person in
the first row is standing in front of someone in the second row. The
heights of the people within each row must increase from left to right,
and each person in the second row must be taller than the person
standing in front of him or her. How many such arrangements of the 6
people are possible?