Lesson: Listing and Counting

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Hi Brent,

I am sorry but I am confused with the question "can be created using digits 2 and 7 only". Does that mean that the 3 digits # has to contain both 2 and 7?


Thanks a lot!
Kate
gmat-admin's picture

Hi Kate,

For that question, we are basically told that the only digits we can choose from are 2's and 7's. It does not mean that every outcome must contain at least one of each digit.

For example, numbers like 222 and 777 meet the give condition.

Cheers,
Brent

Thank you Brent, I felt like I always misinterpret the question and made it harder!! :(

Kate

Hi Brent, I am confused with the answer provided for this question?

How many keystrokes are needed to type numbers from 1 to 1000?

A. 2704
B. 2890
C. 2893
D. 3001
E. None of these

My approach:
For 1-9 - 9 strokes
For 10 - 99 - 90*2 strokes
For 100-999 - 999*3 strokes
For 1000 - 4 strokes

Please help me where am I wrong in this?
gmat-admin's picture

The only problem with your solution is the 3rd step. You have 999 integers from 100 to 999, but there are only 900 integers from 100 to 999.

Here's my full solution: https://gmatclub.com/forum/how-many-keystrokes-are-needed-to-type-number...

Cheers,
Brent

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