Reduce all answers to their lowest terms
20/27 / 10/9

If I am looking at this correctly you have (20/27) divided by (10/9).
The rule for dividing fractions is to keep the first term as it is, change the division to a multiplication, and flip the second term to its reciprocal. Them multiply as usual.
(20/27) * (9/10) = 180 / 270 = 2/3

The density of the cube is 40