trip 1) to 1/4 point & return (leaves 1/2 supply)
trip 2) to 1/2 point & return (picks up 1/4 supply @1/4, leaves 1/2 supply @1/2, picks up 1/4 supply @1/4)
trip 3) to 1/4 point & return (leaves 1/2 supply)
trip 4) to 1/2 point & return (picks up 1/4 supply @1/4, leaves 1/2 supply @1/2, picks up 1/4 supply @1/4)

now at the end of trip 4, there is a full supply @1/2

trip 5) full trip (picks 1/2 suppy @1/2, and 1/2 supply @1/2 on way back)