Example 1. A shopping mall first increased the original price of color TV by 30%, and then wrote "big reward, 20% discount" in the advertisement. As a result, each color TV set earned more than the original price 1 12 yuan. What should be the original price of each color TV?
The equation relationship analysis is: actual selling price-original selling price = 1 12 (yuan).
The original price of each color TV is X yuan.
Solution: x = 2800
A: The original price of each color TV set is 2800 yuan.
Example 2. In order to encourage residents to use electricity, a city electric power company has stipulated the following charging methods: the monthly electricity consumption does not exceed 100 kWh, calculated by 0.5 yuan per kWh; The monthly electricity consumption exceeds 100 kwh, and the excess is calculated as 0.4 yuan per kwh.
(1) If a user paid the electricity bill in 72 yuan in July 2006, how many kWh did the user use in July?
(2) If a user's average electricity bill per kilowatt-hour in August 2006 was 0.45 yuan, how many kilowatt-hours did the user use in August? How much electricity should I pay?
Analysis:
(1) When judging the electricity charge in 72 yuan in July by charging method, the electricity consumption exceeded 100 kWh; (2) According to the calculation of 0.5 yuan > 0.45 yuan > 0.40 yuan, the electricity consumption of users in August exceeded 100 kWh.
The electricity charge for (1) 100 degree is 0.5× 100 = 50 yuan.
Because 72 > 50, the electricity consumption of users in July exceeded 100 degrees. If it exceeds x degrees, then 0.4x = 72-50 and x = 55.
Therefore, the electricity consumption of users in July is 100+55 = 155 (kWh).
(2) The user used X kilowatt-hours of electricity in August, and the electricity bill payable was 0.45x yuan. Because in August, the average electricity charge per kilowatt hour was 0.45 yuan.
< 0.50 yuan, so the electricity consumption in August exceeded 100 degrees. According to the meaning of the question, it is 0.5×100+0.4 (x-100) = 0.45 X.
Solution: x = 200. Then 0.45x = 0.45x200 = 90 (yuan).
Answer: Users used 155 kwh in July and 200 kwh in August, so they should pay 90 yuan electricity bill.
practise
Class 2, Grade 7, Yu Ying Middle School decided to send Xiao Cong and Xiao Ming to buy 22 ballpoint pens and pens * * * and donate them to students in a school in a mountainous area. They went to the mall to see, each 5 yuan for ballpoint pens and each 6 yuan for pens.
(1) If they just spent 120 yuan to buy two kinds of pens, how many pens and ballpoint pens did they buy?
(2) If the ballpoint pen gets a 10% discount and the pen gets a 20% discount, please design a purchase plan on the premise that the required cost does not exceed 100 yuan.
(Reference answer: (1) 12 ballpoint pen and 10 pen; (2) There are different answers, such as 18 ballpoint pen and 4 pens; 19 ballpoint pen, 3 pens, etc. )