A group of girls are riding on a roller coaster.
Girls are on a roller coaster.
A group of girls are riding on a roller coaster so sentence true is also true since being "on" a roller coaster can be seen as riding it.
exists e1 x1 x2 x3 x4.(on(e1,x3) & n_coaster(x3) & of(x3,x4) & n_roller(x4) & Actor(e1,x1) & v_ride(e1) & of(x1,x2) & n_girl(x2) & n_group(x1))
exists p1 x1.(exists x2 x3.(on(x1,x2) & n_coaster(x2) & of(x2,x3) & n_roller(x3)) & n_girl(x1))