If x is the mean proportional between 12.8 and 64.8 and y is the third proportional to 38.4 and 57.6, then 2x : y is equal to:

2 : 3

Mean proportion of 12.8 and 64.8 = x = $\sqrt{12.8\; \times\; 64.8}$     = 28.8

Third proportion to 38.4, 57.6 = y =  $\frac{57.6\; \times\; 57.6}{38.4}$     = 86.4

Then, 2x : y = 57.6 : 86.4 = 2 : 3