Solution: Let the side length of the cube be a, then the volume of the original rectangle is a*a*(a+5).

Cut the volume of 3 decimeters and 2 decimeters high from top to bottom, that is, cut the volume of 5 decimeters. But as you can imagine, the surface area of this 5-decimeter volume is only reduced by four sides, *** 120 square decimeter.

The lateral area of four sides is: a*4*5= 120. The solution is a=6.

Therefore, the volume of rectangular block is: 6*6*(6+5)=396 cubic decimeter.

The best way to do this problem is to give full play to the ability of spatial imagination. The calculation process is not complicated, the key is to think.

Please make it clear, otherwise you can't do it, and you can also make various answers.

3. 10 108 brick.

First of all, we can work out how many bricks there are in a * * *, and then subtract the bricks on the surface to know the number of bricks without lime on the way.

A * * * has: 40*20* 16= 12800 bricks.

The key is to calculate the surface. * * * has six faces. We can calculate every two opposite faces together, but be careful that this calculation will be repeated.

Brick: 40 * 20 * 2+20 * (16-2) * 2+(40-2) * (16-2) = 2692 bricks.

So the brick inside is12800-2692 =10108 brick, which means there is no lime on the drawing.

The key to this problem is to pay attention to the repetition of calculation when calculating the number of tiles. If you are careful, the problem will become simple.

This problem can be solved quickly by the method of equation.

Let this number be x and list the equations according to the meaning of the question:

X+20%*X=270

The solution is X=225.

There is no trick to this topic. Practice makes perfect.

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