Generally the vector to insert molar ratio taken for a ligation reaction is 1:3; and 50 ng of vector is generally used for ligation. Now, since the vector is 2.75 times heavier than the insert (4400/1600 = 2.75), to take thrice the number of molecules of vector present in 50 ng, you need to take 50 x 3/2.75 ng or 54.54 ng of insert.