When I came home from school today, I suddenly saw a shop surrounded by many people. When I took a closer look, it turned out that the store was engaged in promotional activities. You can answer a question by paying one yuan. If you can answer correctly within 30 seconds, you will win the 30 yuan. I saw others losing one after another, and I thought, What's so hard about it? I paid one yuan out of curiosity.

I followed my boss into the store, and he began to ask, "Xiaoming went to the bookstore to buy books, and bought 17 books with 99 yuan money. How much did he spend? " As soon as he finished, he showed a sly look and began to time. As time went on, my sweat dripped down. How to calculate this 99 times 17 without vertical calculation? At this moment, it dawned on me. There is a page in "Daily Practice of Oral Calculations" that says otherwise: The quick calculation method of multiplying a number within 100 by 99 is to multiply this number by 100 first, and then subtract this number. Thinking of this, I blurted out: "1683." As soon as I finished, the boss almost fell to the ground. He looked at the timer for only 28 seconds and gave me the money helplessly.

I walked out of the store with 30 yuan and looked at the envious eyes of others, not to mention how happy I was. At this time, I thought: Mathematics is not only the calculation of 1+ 1, but also has a wide range of uses in life. Besides, math is used for shopping and other things. So, I want to say it again, "Mathematics is really useful."