An IMO subscriber recently asked for help on this question:
Mrs. Margaret weighs 85 lb and her daughter weighs 42 lb less than her. What is the difference of their weights?
85 - 42 = 43 pounds (easy right?!)
WRONG!
Look again; if you read the question carefully, you will notice that the daughter is not 42 pounds, but that she weighs 42 pounds less than her mother who weighs 85 pounds. So the daughter actually weighs 43 pounds, and 42 pounds is their difference in weight. In fact, you don't even have to subtract here at all.
Got you confused? A similar question would be: A book used to cost one dollar but it now costs 10 cents less. What is the difference in the two prices? 10 cents!
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