HELP!!! Physics question on dynamics?

Two blocks are side by side on a frictionless table. One has a mass of '3M' while the other has a mass of '2M'. A horizontal force 'F' is applied on the block with mass '3M'. What is the force that the bigger object exerts on the smaller object?? The answer's 0.4F but how do you do it??? PLEASE HELP!!! thanks!