physics
posted by khikha .
If the aim is to let a 700 kg pump float on the surface of water, how many cylinders ( 50cm*90cm ) containing (( 2/3 from total volume / cylinder )) fresh water should we have ?

The net buoyancy force on a 2/3 full cylinder of water will depend upon the weight of the empty cylinder, which is not specified.

the mass of empty cylinder is 10 kg and the weight is 98 N

How I can let the 700 Kg floats on the surface of water by using cylinders with r=50cm , h=90cm & 2/3 full volume fresh water ? how many cylinders we need ? Is 2/3 full volume better than 1/2 volume ?

In your 50*90 cylinder dimensions, which is the diameter and which is the length? It matters when computing the volume.
The emptier the cylinder, the fewer you need for flotation. Is that also your drinking water supply? 
diameter = 50 cm and length = 90 cm. and the purpose to let the pump float on the surface , while the cylinders under water totally.( top surface of cylinder touching the surface of water )