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Ask everyone about junior high school math problems.
1. An airplane can only fly in the air for 6 hours at most. Its departure and return speeds are 1000 km/h and 800 km/h respectively. How many kilometers should this plane return as far as it can fly?

Suppose this plane should return as far as X kilometers?

x/ 1000+x/800=6

x=8000/3

The plane should return as far as 2667 kilometers.

2. Roll coated paper with a thickness of 0.25mm into a hollow cylinder (ignoring the gap between paper and paper). Its outer diameter is 1.8m, and its inner diameter is 0.25m How long is the rolled coated paper after unrolling? (π takes 3. 14, and the result is accurate to one meter)

(1.8-0.25)/2/0.00025 = 3100 layer

The innermost layer develops to π * 0.25 = 0.785m m.

The outermost layer is π *1.8 = 5.652 m.

The total length is (0.785+5.652) * 3100/2 = 9977m.

This roll of coated paper is 9977 meters when unfolded.

3. Bookstores can make a profit by selling a certain kind of calendars 18 yuan. After two fifths of all calendars are sold, the price of each book is reduced 10 yuan. When all sold out, * * * made a profit of 3000 yuan. How many copies of this calendar did the bookstore sell?

A bookstore * * * sold X calendars.

2/5 * x * 18+3/5 * x *( 18- 10)= 3000

X=250 copies

The bookstore sold 250 calendars.