I may not be a cook, but i know that the number of servings you want is directly proportional to the amount of tomato sauce you need. in other words, if you want half the amount of servings, then you need half the amount of tomato cans.
40% fewer means you want 60% of the servings which means you need 60% of the number of cans need for 1 serving. therefore, you need:
(60/100)(3)
(3/5)(3) so the answer is choice (b)