1. ## Basic Probability

A whole number between 1 and 300 (inclusive) is chosen at random. Find the probability that the number is:

(a) divisible by 3 (b) divisible by 4 (c) divisible by 3 or by 4

So I know that (a) $=\frac{1}{3}$ and (b) $=\frac{1}{4}$, but I was wondering if there's a quick method to find the solution for (c)

2. Originally Posted by Stroodle
A whole number between 1 and 300 (inclusive) is chosen at random. Find the probability that the number is:
(a) divisible by 3 (b) divisible by 4 (c) divisible by 3 or by 4
So I know that (a) $=\frac{1}{3}$ and (b) $=\frac{1}{4}$, but I was wondering if there's a quick method to find the solution for (c)
Well there are 25 numbers in that range divisible by 3 & 4.
$P(3\cup 4)=P(3)+P(4)-P(3\cap 4)$

3. Originally Posted by Plato
Well there are 25 numbers in that range divisible by 3 & 4.
$P(3\cup 4)=P(3)+P(4)-P(3\cap 4)$
Yes, but is there a some kind of method of finding the 25 numbers, without going through all of the even numbers from 0-300?

4. Originally Posted by Stroodle
Originally Posted by Plato
Well there are 25 numbers in that range divisible by 3 & 4.
$P(3\cup 4)=P(3)+P(4)-P(3\cap 4)$
Yes, but is there a some kind of method of finding the 25 numbers, without going through all of the even numbers from 0-300?
If a number is divisible by 3 and divisible by 4, then it has to be divisible by 12. So how many numbers from 1 to 300 are divisible by 12?

01

5. Ahh. Of course. THanks for that!