A class made paper helicopters with four wing designs. Each design was dropped 24 times from a balcony, and the class counted how many drops landed inside a target circle: short wings 14, wide wings 22, long wings 18, folded tips 6. Which design landed inside the circle exactly three-quarters of the time?
Drops landing inside the target circle (24 drops each)
In one kind of bird, each chick is equally likely to be male or female, and whether one chick is male or female has no effect on the others. A nest has three chicks. What is the probability that exactly two of the chicks are female?
A class grew four groups of the same kind of plant for 4 weeks. Each group was placed in either a sunny or a shady spot, and was watered either daily or once a week. The table shows the average height of each group: sunny and daily 24 cm, sunny and weekly 15 cm, shady and daily 13 cm, shady and weekly 11 cm. Which statement is supported by these results?
Spot
Watering
Average height after 4 weeks (cm)
Sunny
Daily
24
Sunny
Once a week
15
Shady
Daily
13
Shady
Once a week
11
AA.
Plants watered daily grew more than twice as tall as plants watered weekly.
BB.
Plants in the shade grew less than half as tall as plants in the sun.
CC.
Watering weekly instead of daily lowered the height by the same amount in sun and shade.
DD.
Moving plants from shade to sun made a bigger difference when they were watered daily.
答案: D
答案：D（Moving plants from shade to sun made a bigger difference when they were watered daily.）
逐条用数据检验。每天浇水对比每周浇水：24对15，13对11，都不到两倍，所以A错。阴处对比阳光处：13大于24的一半，11大于15的一半，所以B错。改为每周浇水后，阳光下的植物矮了24－15＝9厘米，阴处只矮了13－11＝2厘米，所以C错。从阴处移到阳光下，每天浇水的多长了24－13＝11厘米，每周浇水的只多长15－11＝4厘米，所以D有数据支持。
其他选项为什么不对
选项 A（Plants watered daily grew more than twice as tall as plants watered weekly.）：24不到15的两倍，13也不到11的两倍。
选项 B（Plants in the shade grew less than half as tall as plants in the sun.）：13大于24的一半，11也大于15的一半。
选项 C（Watering weekly instead of daily lowered the height by the same amount in sun and shade.）：阳光下矮了9厘米，阴处只矮了2厘米，并不相同。
讲解示例 4
A kayaker paddles a river race and passes four checkpoints. The table shows the distance of each checkpoint from the start and the time the kayaker passed it: Start 0 km at 8:00 am, Gum Creek 14 km at 8:50 am, Old Mill 22 km at 9:14 am, Wattle Bend 38 km at 10:14 am, Finish 45 km at 10:44 am. Between which two checkpoints was the kayaker's average speed greatest?
Checkpoint
Distance from start (km)
Time passed
Start
0
8:00 am
Gum Creek
14
8:50 am
Old Mill
22
9:14 am
Wattle Bend
38
10:14 am
Finish
45
10:44 am
AA.
Start to Gum Creek
BB.
Gum Creek to Old Mill
CC.
Old Mill to Wattle Bend
DD.
Wattle Bend to Finish
答案: B
答案：B（Gum Creek to Old Mill）
算出每一段的距离和时间：起点到Gum Creek是50分钟14公里；Gum Creek到Old Mill是24分钟8公里；Old Mill到Wattle Bend是60分钟16公里；Wattle Bend到终点是30分钟7公里。换算成每小时多少公里：16.8、20、16和14。最快的一段是Gum Creek到Old Mill（24分钟8公里，即每3分钟1公里，时速20公里）。
其他选项为什么不对
选项 A（Start to Gum Creek）：50分钟14公里，时速16.8公里，比20公里慢。
选项 C（Old Mill to Wattle Bend）：这一段距离最长，但用了整整1小时，时速只有16公里。
选项 D（Wattle Bend to Finish）：30分钟7公里，时速14公里，是最慢的一段。
