Answer:
2/5 g/cm
Step-by-step explanation:
When you want to know "A per B", divide the given quantity of A by the corresponding quantity of B. ("Per" essentially means "divided by".)
It can be convenient to choose table values that make the division easy:
12 g/(30 cm) = 4/10 g/cm = 0.4 g/cm
20 g/(50 cm) = 2/5 g/cm . . . . . . . . . . . . . same as 0.4 g/cm
