1. Think of a number as a three-digit number with two zeros in front, for example, think of 1 as 00 1. Think of a two-digit number as a three-digit number, preceded by a zero. For example, consider 1 1 as 0 1 1. Then all natural numbers from 1 to 500 can be regarded as "three digits". In addition to 500, consider such a "three-digit" without 4. One hundred yuan has 0 and 1. There are nine choices for ten digits: 0, 1, 2, 3, 5, 6, 7, 8, 9. Unit, there are nine ways to choose. Therefore, besides 500, there are c (1, 4) × c (1, 9) × c (1, 9) = 4× 9× 9 = 324 "three digits" without 4. take notice of

2. This is actually the problem of finding the common multiple. First, according to the calculation of 300 meters by 5 meters, 60 pits are needed.

However, 30 pits have been dug before, and it depends on whether it can be dug like a 5-meter pit. The least common multiple of No.3 pit and No.5 pit is 15 90/ 15=6, which means that the six pits dug before can still be used if dug again, so the answer is 60-6=54.

3. A cycle has15 *14/2 =105 combinations.

That is 105 8 hours.

105*8/3=35 days

4.A =4/ 13，5/6 A = B，4/ 1 1 (A+B) = C，D-C =4000。

The substitution method gives 156000. If there is an option, you don't need to calculate it carefully, just eliminate the interference term.

A.18.6 million B.15.6 million C.210.8 million D.22.3 million

[answer] b

[Resolution] The population of Area A is the population of the whole city (4/ 13), so the population of the whole city is a multiple of 13. Combination options, select B.

5. The answer is 15. This is a variation of a product sequence. Law: (item 1-3) × (item 2-3) = item 3.