A school club makes a booklet by stacking 9 sheets of paper, folding the whole stack in half and stapling along the fold. Each sheet becomes 4 pages, and the pages are numbered 1 to 36 in order. When the booklet is pulled apart, one sheet has page 7 printed on it. What is the largest page number printed on that same sheet?
A nature guide uses this rule for one kind of cricket: count the chirps in 25 seconds, divide by 3, then add 4 to estimate the air temperature in °C. One evening the temperature is 22 °C. About how many chirps would the guide expect to count in 25 seconds?
Three gear wheels sit in a row, each touching the next. Gear A on the left has 12 teeth, gear B in the middle has 18 teeth and gear C on the right has 8 teeth. Gear A is turned clockwise through exactly 2 full turns. What does gear C do?
AA.
It makes 3 full turns clockwise.
BB.
It makes 3 full turns anticlockwise.
CC.
It makes 2 full turns clockwise.
DD.
It makes 1⅓ turns anticlockwise.
答案: A
答案：A（It makes 3 full turns clockwise.）
相互咬合的齿轮转向相反：A顺时针，B就逆时针，C再被反过来，变成顺时针。A转2圈，每个咬合处都有2×12＝24个齿经过。C有8个齿，24÷8＝3，所以C顺时针转3整圈。
其他选项为什么不对
选项 B（It makes 3 full turns anticlockwise.）：圈数对了，但C被反转了两次，所以和A同方向。
选项 C（It makes 2 full turns clockwise.）：以为每个齿轮转的圈数都一样，忽略了齿数不同。
选项 D（It makes 1⅓ turns anticlockwise.）：这是中间齿轮B的转动（24÷18＝1⅓圈，逆时针），不是C。
讲解示例 4
A cook counts that 40 grains of rice weigh 1 gram. A school canteen is open only on school days, 5 days a week, and it uses one 10 kg sack of rice every 2 days that it is open. About how many grains of rice does the canteen use in 4 weeks?
A class tied a weight to strings of different lengths to make pendulums. For each length they timed 10 complete swings. Their results were: 25 cm took 10 seconds, 50 cm took 14 seconds, 100 cm took 20 seconds and 200 cm took 28 seconds. Following the pattern in the results, about how long would 10 swings take for a pendulum 400 cm long?
A packet of seeds says that, on average, 3 out of every 4 seeds sprout. Ana wants at least 36 seedlings for the school garden. What is the smallest number of seeds she should plant so that she can expect at least 36 seedlings?
In a science lab, a dish holds 300 bacteria at 9:00 am. The number of bacteria doubles every 20 minutes. At what time will the dish first hold more than 10 000 bacteria?
A garden designer marks out a garden bed in the shape of a regular hexagon. Straight paths join every second corner of the hexagon, so the three paths form a triangle inside it. The triangle is planted with lawn, and the three pieces outside the triangle are planted with flowers. What fraction of the whole bed is lawn?
