Density is defined as the mass per volume. In problems 1, 2, and 4, you will be dividing the mass by the volume to determine density. Problem 3 asks for weight, so you will be multiplying volume by density to determine mass.
#1
40.5 g / 15 ml = 2.7 g/ml
#2
306.0 g / 22.5 ml = 13.6 g/ml
#3
0.789 g/ml * 200 ml = 157.8 g
#4
1896 g / 212.52 ml = 8.92 g/ml
(Since 1 cubic cm = 1 ml, we can find the volume by multiplying the lengths of the block's sides: 8.4 cm * 5.5 cm * 4.6 cm = 212.52 cm^3 = 212.52 ml)