Postal Worker

A postal worker has 600 houses on his route. During one snowy day, he noticed that every other house’s mailbox was blocked by snow, and he could not deliver the mail. He also noticed that he had no mail to deliver to every third house on the route. By the end of his route, how many households had mail that was still in his truck?
Source: NCTM Mathematics Teacher 2008

SOLUTION
Suppose there are 18 households on the route.


B = blocked by snow
N = no mail
Number of households that have no mail, 18\div 3=6, namely \{3, 6, 9, 12, 15, 18\}.
Number of households that have mail, 18-6=12, namely {1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, 17}.
Half of the 12 households having mail are blocked by snow; the mail of 6 households is still in his truck.
Number of households that have no mail, 600\div 3=200.
Number of households that have mail, 600-200=400.
Half of the 400 households having mail are blocked by snow; the mail of 200 households is still in his truck.

Answer: 200

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 )

Twitter picture

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

Facebook photo

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

Google+ photo

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

Connecting to %s