It is the Achilles' heel of a lot of math students: word problems. It doesn't matter how well you know the formulas; if you can't decipher what the word question is asking, you can't answer the question. I see this all too often.
The trick to changing a word problem into mathematics is to look for keywords. There are too many keywords to list them all here, but these are some of the more common words and what they often mean:
"is" or "if" means "equals"
"and" or "sum" or "more" means "addition"
"difference" or "change" or "less" means "subtraction"
"product" or "each" means "multiplication"
"quotient" or "ratio" or "fraction" means "division"
"at least" or "only" means "less than or equal to"
"more than" means "greater than (or equal to)"
For Calculus: "marginal" or "instantaneous" or "tangent" means "derivative"
For example, "I bought shirts and pants at the store. The shirts cost $10 each and the pants cost $15 each. How many shirts and pants can I buy if I have only $50?" translates to: 10s + 15p = 50. In this case, the keywords were "and", "each", "if", and "only".
Once you get used to looking for the keywords, the word problems can become much simpler.
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