so we need finally to have the result as Fixed Costs /(C/S) hence 648000/(C/S)
C/S we know such as 0.27 / 0.56 / 0.38 Sales mix we know as 3:2:5 weighted average contribution is (3*0.27+2*0.56+5*0.38) / total sales (3+2+5) = 0.383 this is C/S coef.
Finally we should have 648 000 / 0.383 = 1 691 609 ( please consider decimal numbers, it is really impacting).