Lesson: Listing and Counting

Comment on Listing and Counting

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

ari.banerjee's picture

Hi Brent,

Are these questions, listed below the videos, OG questions?

Im planning to do the OG qs in the end so I was curious.
gmat-admin's picture

Good question, Ari.
The source for each question is listed on the right side of the link.
When I say the source is GMAT Club, then either the original poster didn't note the source or it's an original question that was created by GMAT Club staff.
So, in the above links there are no official questions. That said, I often post unofficial questions if they closely resemble official GMAT questions.

Cheers,
Brent

ari.banerjee's picture

Thank you.

That makes sense. Mind Boggling qs nonetheless.

For some reason when I solve the OGs I dont encounter such high levels of difficulties. Are OG qs mostly easy with a few 700 level qs inserted here and there?
gmat-admin's picture

I think you're right. Many non-official questions often push the limit difficulty-wise.

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