10 cm^3 of a normal specimen of human blood contains 1.2 g of hemoglobin. how many grams does 39 cm^3 of the same blood contain? 39 cm^3 of blood contains grams of hemoglobin.

This is a question of direct proportionality. The mass of hemoglobin is expected to increase with increase in volume of the blood.
Therefore;

10 cm^3 of blood 1.2 g of hemoglobin
39 cm^3 of blood x    g of hemoglobin

The,
x = (39*1.2)/10 = 4.68 g of hemoglobin.

That is, 39 cm^3 of blood contains 4.68 grams of hemoglobin.