The tour guide should have rented 16 vans to accommodate the group of 140 tourists.
We multiply the number of vans to the number of tourists it can hold.
15 * 9 = 135 tourists
Out of the 140 tourists, only 135 can be accommodated by the 15 vans rented leaving 5 tourists with no ride.
140 - 135 = 5 tourists
Had the tour guide rented 16 vans, everyone would have a ride, leaving 4 extra seats unoccupied.
16 * 9 = 144 tourists capable of being seated
144 - 140 (actual number of tourists) = 4 extra seats available.