What is the average number of Samsung cellular phones sold by stores B, C and D?

% of cellular phones (both nokia and samsung) sold by stores B,C and D together = (21 + 15 + 33)% = 69%

=> Number of cellular phones (both nokia and samsung) sold by stores B,C and D together

= $$\frac{69}{100} \times 6400 = 4416$$

% of nokia phones sold by stores B,C and D together = (18 + 20 + 30)% = 68%

=> Number of nokia phones sold by stores B,C and D together

= $$\frac{68}{100} \times 3000 = 2040$$

=> Total number of Samsung cellular phones sold by stores B, C and D = 4416 - 2040 = 2376

$$\therefore$$ Required average = $$\frac{2376}{3} = 792$$