Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized for You

we will pick new questions that match your level based on your Timer History

Track Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

It appears that you are browsing the GMAT Club forum unregistered!

Signing up is free, quick, and confidential.
Join other 500,000 members and get the full benefits of GMAT Club

Registration gives you:

Tests

Take 11 tests and quizzes from GMAT Club and leading GMAT prep companies such as Manhattan GMAT,
Knewton, and others. All are free for GMAT Club members.

Applicant Stats

View detailed applicant stats such as GPA, GMAT score, work experience, location, application
status, and more

Books/Downloads

Download thousands of study notes,
question collections, GMAT Club’s
Grammar and Math books.
All are free!

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

Do you guys know any trick to make such a calculation?

Thanks!

i am not aware of any nice shortcut. however, sometime the answer choices help in that given these you can find the right answer quickly. for example the units digit of the answer can be quickly calculated (it is 3), so if you had 5 answer choices and only one of those had 3 in its unit digit - you could have your answer very quickly.

Do you guys know any trick to make such a calculation?

Thanks!

i am not aware of any nice shortcut. however, sometime the answer choices help in that given these you can find the right answer quickly. for example the units digit of the answer can be quickly calculated (it is 3), so if you had 5 answer choices and only one of those had 3 in its unit digit - you could have your answer very quickly.

from 5! (i.e. 5!, 6!, 7! ...) the units digits is 0
so the units digit of the sum 1!+...+20! depends only on the units digits of 1!+2!+3!+4! = 1+2+6+24 = 33
hence the units digits of the complete sum is 3
similarly you can relatively easily compute the tens digit of the sum (you need to check only 1!...9!) and it will be inevitably and odd digit (or more specifically 1 if i'm not wrong with my quick no-paper calculations)

this would probably suffice to choose the right answer from a reasonable set of choices...

Do you guys know any trick to make such a calculation?

Thanks!

although no shortcut .... it seems that this sum may be part of a "bigger picture" question - maybe related to counting/combinatorics....
if so, it might be that the wider context situation has a shortcut that bypasses the sum of factorial altogether.

[quote="hobbit"][quote="querio"]What is 1!+2!+...+20! ?

Do you guys know any trick to make such a calculation?

Thanks![/quote]

although no shortcut .... it seems that this sum may be part of a "bigger picture" question - maybe related to counting/combinatorics....
if so, it might be that the wider context situation has a shortcut that bypasses the sum of factorial altogether.[/quote]

Yeah, you're right... I don't think Gmat will ask to make the calculation just to get the final number, but it will somehow hide the answer among the answer choices. Thanks!

GMAT will not ask questions such as the sum of factorials to 20. Although, it is possible that they may ask the units digit of 1!+2!+3! .......+ 20! as described @querio. Instead, the types of questions that I have seen on the official GMAT follow the pattern of the examples listed below:

1) How many terminating zeroes does 100! have when written as an integer? (Answer is 24)

2) What is the largest prime factor of 13! - 11! ? (Answer is 31)

Cheers, Dabral

gmatclubot

Re: PS - sum of factorials
[#permalink]
09 Jun 2014, 16:44

This is the kickoff for my 2016-2017 application season. After a summer of introspect and debate I have decided to relaunch my b-school application journey. Why would anyone want...

Check out this awesome article about Anderson on Poets Quants, http://poetsandquants.com/2015/01/02/uclas-anderson-school-morphs-into-a-friendly-tech-hub/ . Anderson is a great place! Sorry for the lack of updates recently. I...

According to the Nebula Award categories, a novel must be over 40,000 words. In the past year I have written assignments for 22 classes totaling just under 65...