# Grade 1: “Use a 100 note to buy item 35, how much money will you get back?”

A man surnamed Wang (China) has a son who is in grade 1. One day, the boy returned from school with a test that the teacher crossed wrong and asked his father to answer.

The problem requires the following: “Mr A used 100 yuan to buy an item worth 35 yuan. How much money did Mr. A get back?”.

The boy quickly wrote down the answer, and said the answer was 100 – 65 = 35that is, subtract the value of the item from the amount of money that Mr. A brought.

However, the answer was crossed out by the teacher and no points were given. As a person who values ​​his children’s academic achievements, Mr. Wang got angry, demanded to meet the teacher directly and asked for an explanation.

The teacher replied: “My son didn’t do anything wrong. However, he only did one of the four situations of the answer.” After that, the teacher took a paper and wrote down 3 remaining cases that the boy had not thought of:

– Case 1: If Mr. A has 2 bills of 50 yuan. You just need to give a 50 yuan bill to the shopkeeper and get 15 yuan back (50 – 35 = 15).

– Case 2: If Mr. A has 5 20 yuan bills. You just need to give 2 20 yuan bills to the shopkeeper and get 5 yuan back (2×20 – 35 = 5).

– Case 3: If Mr. A has 1 50 yuan bill, 1 20 yuan bill, 1 10 yuan bill and 2 5 yuan bills, he only needs to give 1 20 yuan bill, 1 10 yuan bill and 1 5 yuan bill to the seller and get nothing back (20 + 10 + 5 – 35 = 0).

However, after listening to the female teacher’s explanation, Mr. Wang still disagreed with the results of this problem. “Is this an assignment for 1st graders, or is it a quiz question?” – Mr. Wang angered.

Many people also agree with Mr. Wang’s comment on the above problem. The point of controversy here is that the topic asked Mr. A to use 100 yuan to buy things, but the teacher wanted the students to have to sort the 100 yuan bill. And of course, a problem is not clear in the way of problem solving, lack of necessary and sufficient data, how can students fill in the correct answer?

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