The corridor 20 m long and 5 m wide was shortened by 3 m in length. How much has the corridor area decreased?

20×5 = 100 is the initial area of the corridor.
20-3 = 17 – the length of the corridor has become, the width has not changed. Hence,
17×5 = 85 – the area of the corridor has become.
100-85 = 15.
Answer: 15

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