a rectangular swimming pool is 10 m long, 5 m wide and 6 m high. how much water is needed to fill it up to three-fourths of its height

3/4 of height is 3/4*6=4.5m. Volume to fill 10*5*4.5=225 cu m. 1 ml of water=1cc volume. 1 cu m=1,000,000cc.

225cu m=225,000,000cc=225,000,000ml=225,000 litres.

So you would need 225,000 l of water.