4-Digit Numbers Divisible by 7

Given the digits 1,3,6, and 9, find the probability that a four-digit number formed by using each of them only once is divisible by 7.
Source: NCTM Mathematics Teacher, February 2008

Solution
There are 4\times 3\times 2\times 1=24 possible four-digit numbers using 1,3,6,9 each only once
1369,1396,1639,1693,1936,1963
3169,3196,\underline{3619},3691,3916,3961
\underline{6139},6193,6319,\underline{6391},6913,6931
9136,\underline{9163},9316,9361,9613,9631
Numbers divisible by 7 are 3619,6139,6391,9163
Probability = 4/24=1/6.

Answer: 1/6

Advertisements

About mvtrinh

Retired high school math teacher.
This entry was posted in Problem solving and tagged , , , , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s