We have that every background can be combined to any of the poses and each background yields a unique pose. So, for each unique background we get a 5 different photographs. Since there are 4 backgrounds, we get in total 4*5=20 photos. This can be generalized in other problems too; for example if there are 3 dinner dishes and 6 lunch dishes on a menu, there are 3*6=18 combinations.
