# Probability involving two digit numbers.

• Jan 24th 2011, 09:58 AM
haftakhan
Probability involving two digit numbers.
A two-digit number is written down at random.Find the probability that the number will be

A)Smaller than 20
B)Even
C)Multiple of 5

How to solve this question ?
• Jan 24th 2011, 10:01 AM
wonderboy1953
Quote:

Originally Posted by haftakhan
A two-digit number is written down at random.Find the probability that the number will be

A)Smaller than 20
B)Even
C)Multiple of 5

How to solve this question ?

I don't know what random would mean in this case as there must be a reason why the two-digit number was selected.
• Jan 24th 2011, 10:02 AM
Quote:

Originally Posted by haftakhan
A two-digit number is written down at random.Find the probability that the number will be

A)Smaller than 20
B)Even
C)Multiple of 5

How to solve this question ?

Numbers do not start with 0, but can end with 0 and non-zero digits may repeat.

Hence there are 9(10) 2-digit numbers.

How many are <20 ?
How many end in an even digit ?
How many end with a 5 or 0 ?
• Jan 24th 2011, 10:07 AM
JesseElFantasma
Quote:

Originally Posted by haftakhan
A two-digit number is written down at random.Find the probability that the number will be

A)Smaller than 20
B)Even
C)Multiple of 5

How to solve this question ?

If it is lower than 20 you know that it is either 19, 18, 17, 16.....
If it is even it is 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20.
Multiples of five lower than 20 that are even, well there is only 1: 10
• Jan 24th 2011, 10:40 AM
HallsofIvy
Quote:

Originally Posted by JesseElFantasma
If it is lower than 20 you know that it is either 19, 18, 17, 16.....
If it is even it is 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20.
Multiples of five lower than 20 that are even, well there is only 1: 10

You appear to be assuming that these are "cumulative"- that is, that the "B) even" means "even and less than 20" and that "C) a multiple of 5" means "a multiple of 5 and even and less than 20". I don't believe those assumptions are warrented.
• Jan 24th 2011, 01:33 PM
mr fantastic
Quote:

Originally Posted by haftakhan
A two-digit number is written down at random.Find the probability that the number will be

A)Smaller than 20
B)Even
C)Multiple of 5

How to solve this question ?

The obvious but blunt approach is to count all the two digit numbers that satisfy each condition and then divide by the total number of two digit numbers.