ok
density = mass/volume
therefore, mass = density x volume
so if you have 20ml of acetone you have 20 x 0.787 g
divide this by the molar mass of acetone to get moles
= 15.68/58 = 0.270 moles
this number of moles in 200ml is the same concentration as 0.270/0.2 moles/litre
= 1.352 Molar