Why liquid have a definite volume but no definite shape?

This is because the intermolecular forces are strong enough to hold the molecule together but not so strong as to fix them into definite positions (as in solids) instead, they possess fluidity and hence no definite shape.

Volume of a cylinder containing 10 marbles and gas is 1 litre at 2 atm pressure. Now pressure on the cylinder is increased to 4 atm, at which volume becomes 725 ml. Calculate volume of each marble.the