Solution: Wood cover diameter: 0.45+0.05 = 0.5 (m)
Radius of wood cover: 0.50 ÷ 2 = 0.25 (m)
Wood coverage area: 3.14× 0.25× 0.25 = 0.19625 (square meter) = 19.625 square decimeter.
The circumference of the wooden cover, that is, the length of the iron sheet: 3.14× 0.5 =1.57 (m) =15.7 decimeter.
Answer: The area of the wooden cover is 19.625 square decimeter; If an iron sheet is nailed to the outer edge of the wooden cover, the length of the iron sheet should be at least 15.7 decimeter.
2. Add 2g of salt and 10g of water into 50g of salt water with a salt content of 20%. What is the salt content of salt water at this time?
Solution: (50× 20%+2) ÷ (50+2+10) =12 ÷ 62 ≈19.35%.
Answer: At this time, the salt content of brine is about 19.35%.
3. Cut a wire with a length of 188.4 cm into two sections, the short one is 20% of the long one, and then form two circles respectively. How many square centimeters is the area of the great circle?
Solution: the circumference of the great circle:188.4÷ (1+20%) =157 (cm).
Radius of great circle: 157 ÷ 3. 14 ÷ 2 = 25 (cm).
Great circle area: 3.14× 25× 25 =1962.5 (square centimeter)
A: The area of the Great Circle is 1962.5 square centimeters.
Three piles of coal weigh 24 tons. If 0.5 ton is transported from the first and second piles to the third pile, the weight ratio of these three piles of coal is 2: 1: 3. How many tons did the original three piles of coal weigh respectively?
Solution: 2+ 1+3 = 6.
After transportation, the three piles of coal are respectively: 24× 2/6 = 8 (tons); 24× 1/6 = 4 (ton); 24× 3/6 = 12 (ton)
The original three piles of coal are: 8+0.5 = 8.5 (tons); 4+0.5 = 4.5 (ton); 12-1=11(ton)
Answer: The original weight of the first coal pile was 8.5 tons, the original weight of the second coal pile was 4.5 tons, and the original weight of the third coal pile was 1 1 ton.
5. A bus leaves from the main station, and 1 seats on the bus are empty. 0/2 people get off at Station A/KLOC-and 20 people get on. At this point, the seats in the car are just full. How many seats are there in this car?
Solution: (20- 12) ÷ 1/6 = 48 (pieces)
This car has 48 seats.
6. The ratio of the diameters of two circles is 3: 2. Assuming that the radius of the great circle is 6 decimeters, how many square decimeters is the area of the small circle?
Solution: Great circle diameter: 6× 2 = 12 (decimeter)
Diameter of small circle: 12× 2/3 = 8 (decimeter)
Radius of small circle: 8 ÷ 2 = 4 (decimeter)
Small circle area: 3. 14×4×4≈5. 10 (square decimeter)
A: The area of the small circle is about 5. 10 square decimeter.
7. There are a pair of meshing gears on the machine tool. The radius of the big gear is 5cm, and the radius of the small gear is 2cm. The big gear rotates 10 cycle, and how many cycles does the small gear rotate?
Solution: (3.14× 5× 2) ÷ (3.14× 2× 2) = 2.5 (weeks)
A: The pinion rotates for 2.5 cycles.
8. At the beginning of a class, 60% of the students meet the physical education standards, and 18 of the students fail to meet the standards. By the end of the period, 15 people reached the standard. What percentage of students in this class reached the standard at the end of the term?
Solution: Class size: 18 ÷ (1-60%) = 45.
The final compliance rate: (45× 60%+ 15) ÷ 45 ≈ 93.33%.
A: About 93.33% of the students in the final class reached the standard.
8. The library of Wang Jiacun Village Committee has 630 books on science, technology and literature, of which 20% are science and technology books. Later, I bought a batch of science and technology books. At this time, science and technology books account for 30% of the two books. How many science and technology books did you buy later?
Solution: Suppose you buy X more science books, then
(630+x)×30%=630×20+x
Solution: x = 90
A: I bought another 90 science and technology books.