2005 PMWC Problems

Problem 1

What is the greatest possible number one can get by discarding $100$ digits, in any order, from the number $123456789101112 \dots 585960$ ?

Solution

Problem 2

Let $\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=\frac{1}{2005}$ , where $a$ and $b$ are different four-digit positive integers (natural numbers) and $c$ is a five-digit positive integer (natural number). What is the number $c$ ?

Solution

Problem 3

Let $x$ be a fraction between $\frac{35}{36}$ and $\frac{91}{183}$ . If the denominator of $x$ is $455$ and the numerator and denominator have no common factor except $1$ , how many possible values are there for $x$ ?

Solution

Problem 4

Solution

Problem 5

Consider the following conditions on the positive integer (natural number) $a$ :

1. $3a + 5 > 40$

2. $49a \ge 301$

3. $20a \le 999$

4. $101a + 53 \ge 2332$

5. $15a – 7 \ge 144$

If only three of these conditions are true, what is the value of $a$ ?

Solution

Problem 6

A group of $100$ people consists of men, women and children (at least one of each). Exactly $200$ apples are distributed in such a way that each man gets $6$ apples, each woman gets $4$ apples and each child gets $1$ apple. In how many possible ways can this be done?

Solution

Problem 7

How many numbers are there in the list $1, 2, 3, 4, 5, \dots, 10000$ which contain exactly two consecutive $9$ 's such as $993, 1992$ and $9929$ , but not $9295$ or $1999$ ?

Solution

Problem 8

Some people in Hong Kong express $2/8$ as 8th Feb and others express $2/8$ as 2nd Aug. This can be confusing as when we see $2/8$ , we don’t know whether it is 8th Feb or 2nd Aug. However, it is easy to understand $9/22$ or $22/9$ as 22nd Sept, because there are only $12$ months in a year. How many dates in a year can cause this confusion?

Solution

Problem 9

There are four consecutive positive integers (natural numbers) less than $2005$ such that the first (smallest) number is a multiple of $5$ , the second number is a multiple of $7$ , the third number is a multiple of $9$ and the last number is a multiple of $11$ . What is the first of these four numbers?

Solution

Problem 10

A long string is folded in half eight times, then cut in the middle. How many pieces are obtained?

Solution

Problem 11

There are 4 men: A, B, C and D. Each has a son. The four sons are asked to enter a dark room. Then A, B, C and D enter the dark room, and each of them walks out with just one child. If none of them comes out with his own son, in how many ways can this happen?

Solution

Problem 12

Solution

Problem 13

Sixty meters of rope is used to make three sides of a rectangular camping area with a long wall used as the other side. The length of each side of the rectangle is a natural number. What is the largest area that can be enclosed by the rope and the wall?

Solution

Problem 14

On a balance scale, three green balls balance six blue balls, two yellow balls balance five blue balls and six blue balls balance four white balls. How many blue balls are needed to balance four green, two yellow and two white balls?

Solution

Problem 15

The sum of the two three-digit integers, $\text{6A2}$ and $\text{B34}$ , is divisible by $18$ . What is the largest possible product of $\text{A}$ and $\text{B}$ ?