Question

How many different ways can 2 students be seated in a row of 7 desks , so that there is always at least one empty desk between the students

Answer 1

3 ways to seat the students:
with two empty seats between
1 empty w/ one student on the left most
1 empty....right most

two students can be interchanged

3x2=6

Answer 2

nCr = n!/ (n-r)!*r!

we have n = 4 and r = 2

nCr= 4*3*2*1/ ( 4-2) ! * 2 *1 
= 4*3/ 2*1 = 6

However sometimes it is easier to find out the soln. using the basic method as shown by GMAT-HELP.