Fist you have to find the volume which is 24 x 18 x 5= 2160 feet.
Then the dump truck can only take 8 cubic yards so then you have to make the 2160 feet into yards.
There are 3 feet in one yard so 2160/3=720
so now there are 720 yards and since the dump truck can only take 8 yards a trip then it would take 90 trips/dumps to empty out the area for the pool.
720/8=90
Hope this helps