Have your students covered division and multiplication yet? To me, these problems can all be solved using the "meaning of division" and the "meaning of multiplication".
"Meaning of multiplication": If I have n groups of equal size m, then I have n*m total objects.
"Meaning of division": There are actually two!
(a) If I have m objects which are partitioned into n equally sized groups, then I have m/n objects in each group (this is the "how much in each group" interpretation of division).
(b) If I have m objects, and I put these objects into equally sized groups where each group has n objects in it, then I have m/n groups of objects (this is the "how many groups" interpretation of division).
Here are the answers to your questions from this perspective:
Every 100g of soup contains 150kj of energy. Call 100g of soup a "portion". I have a "serving" of soup which contains 450 kj of energy. So, if I want to see how many 100g portions of soup are in one serving, I need to see how many groups of 150kj there are in 450kj. The answer to this question (by "the meaning of division") is 450/150 = 3 portions. Then, since each of these 3 equally sized portions of soup is 100g, (by "the meaning of multiplication") I have 3*100g = 300g of soup.
4cm represents 8mm. So I have 4 centimeters, which, in total, represent 8 millimeters. How many millimeters does one centimeter represent? Each centimeter represents an equal sized group of millimeters. So I have 4 equally sized groups of millimeters. Thus each group has 8/4 = 2 millimeters in it. So each centimeter represents 2 millimeters.