BTGmoderatorLU wrote: ↑

Sat May 13, 2023 6:34 am

Source: Magoosh

A “descending number” is a three-digit number, such that the units digit is less than the tens digits and the tens digit is less than the hundreds digit. What is the probability that a three-digit number chosen at random is a “descending number”?

A) \(3/25\)
B) \(2/9\)
C) \(2/15\)
D) \(1/9\)
E) \(1/10\)

The OA is C

Let's use some counting methods to solve this.

P(selected number is "descending") = total # of descending numbers/total # of 3-digit numbers

total # of 3-digit numbers
3-digit numbers go from 100 to 999 inclusive
A nice rule says: the number of integers from x to y inclusive equals y - x + 1
999 - 100 + 1 = 900

total # of descending numbers
First recognize that, in a descending number, all 3 digits are different (due to the condition that units digit < tens digit < hundreds digit)
So, let's first choose 3 different digits (from 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9)
Since the order in which we choose the numbers does not matter, we can use COMBINATIONS.
We can select 3 digits from 10 digits in 10C3 ways (= 120 ways)
If anyone is interested, the video below shows you how to calculate combinations (like 10C3) in your head

IMPORTANT: At this point, we've selected 120 sets of 3 different digits.
Since there's only 1 way to arrange those 3 digits in descending order, we can say that, for each set of 3-different digits, there is exactly ONE descending number,
For example, let's say one of the 3-digit sets is {5, 1, 8}. There is only one way to arrange these 3 digits into a descending number (158)
Since we have 120 different 3-digit sets, there must be 120 descending numbers

So, P(selected number is "descending") = 120/900
= 2/15

Answer: C

RELATED VIDEO: https://www.youtube.com/watch?v=05a0A7vjG8I