Question

A liquid has a density of 1.5 g/cc. 50 cc of it is mixed with 30 cc of water . What is the density of mixture ?

Answer 1

density of liquid =  1.5 gm/cc
volume mixed = 50 cc 
   => mass of liquid mixed = 1.5 * 50 = 75 gms

density of water = 1 gm/cc
volume mixed = 30 cc
   => water mixed = 3 0 * 1 = 30 gm

Total  volume of mixture =  75 + 30 = 105 gms

Density of mixture = 105 gm / 80 cc =  1.3125 gms/cc

BY FORMULA:

   density of a mixture =  [ V1 * d1 + V2 * d2 ] / (V1 + V2)
               = [ 50 * 1.5 + 30 * 1 ] / (50 + 30)
               =105 gm/ 80 cc
               = 1.3125 gm/cc

Answer 2

Density of the liquid (da) = 1.5 g/cc
Density of water (dw) = 1 g/cc
Volume of the liquid (va) = 50 cc
Volume of water (vw) = 30 cc

Density = mass / volume 
⇒ Mass = density × volume

So,
mass of the liquid (ma) = da × va
                                        = 1.5 × 50
                                        = 75 g
mass of water (mw) = dw × vw
                              = 1 × 30
                              = 30 g
Total mass = ma +mw
                 = 75 + 30 
                 = 105 g
Total volume = va +vw
                    = 50 + 30
                    = 80 cc

Density of mixture = total mass / total volume
                            = 105 / 80
                            = 1.3125 gm/cc