A solution contains to 40g of common salt and 320g of water calculate the concentration?

40g/320g=0.125

so, that means there is 0.125g of salt per 1mL of water. (0.125g/mL)
Generally,when dealing with water, mL and grams are directly interchangable, so 320g is also 320mL
For future reference, you divide the solute(salt) by the solution(water). Make sure they are in the same units, both in grams for example, not one in grams on in liters. If they are you convert them to the same unit then do the division.
Cheers