







Factorials 
20170727 

From dinesh: Is there any shortcut formula for multiply series numbers.
like
1x2x3x4x5.......x100 =? Answered by Penny Nom. 





An exercise with factorials 
20160207 

From vidhi: find the value of n: (11/2) (22/3) (33/4)...(1515/16) = n!/16 Answered by Penny Nom. 





Factorials 
20140606 

From penny: What is factorial? For eg. Like 2!, 3! 4! Etc. Answered by Penny Nom. 





(11/2)(22/3)(33/4)...(1515/16)=n!/16 
20110215 

From Fiona: Could you help me find the value of n: (11/2)(22/3)(33/4)...(1515/16)=n!/16 Answered by Penny Nom. 





Factorials 
20100323 

From Leah: When should you use factorials? Answered by Robert Dawson. 





The tens digit of 0! + 1! + 2! + 3! + ... + 2000! 
20091221 

From Alicia: What is the tens digit of 0! + 1! + 2! + 3! + ... + 2000!? Answered by Robert Dawson and Penny Nom. 





The last non cero digit of a factorial 
20090612 

From Wilson: How can I find the last non cero digit from a factorial calculation of a big number.
For example 10! = 3628800, the last non cero digit is 8.
What is the last non cero digit of 10! ?? Answered by Robert Dawson. 





The sequence 2n!  1 
20090201 

From Penny: I am trying to help my son with this problem. Find the first five terms of the sequence that can be written from the formula A= 2n !1. Answered by Penny Nom. 





The tens digit of 0! + 1! + 2! + 3! + 4! .............+9999! + 10000! 
20090123 

From Kaylyn: determine the tens digit for the expression:
0! + 1! + 2! + 3! + 4! .............+9999! + 10000! Answered by Robert Dawson and Penny Nom. 





n choose r equals n1 choose r plus n1 choose r  1 
20080714 

From fae: Prove that
( n ) = ( n – 1) + ( n  1 )
( r ) ( r ) (r1)
NOTE: the ( ) should be one for n taken r and so on. but there is no one big ( ) that will cater two lines Answered by Janice Cotcher. 





Sum of factorials 
20080126 

From Emily: What is the tens digit in the sum 7! + 8! + 9! ... + 2006! Answered by Stephen La Rocque. 





Finding the last nonzero digits of large factorials 
20071004 

From Mukesh: i have to find last five non zero digits of integer which can be very large (
upto 10^12) . i can find last non zero digit of of any factorial. Now my problem is that
i have to find last five non zero digit of factorial and also i want to general method for
last K non zero digits of factorial n. For example 10!=3628800 so last non zero digit is 8 ,last two
non zero digit is 88 .....and last five non zero digit is 36288. Answered by Victoria West. 





Factorial fraction 
20070803 

From Sekhoane: Expand completely: (N2)!/N!(9N1)! Answered by Stephen La Rocque. 





Combinations 
20070509 

From Michael: Show that 5 X C(n,5) = n X C(n1,4) Answered by Stephen La Rocque. 





Probability of getting an A 
20070509 

From Christine: In a class of 15 people, exactly 3 got an A. If 2
people are randomly chosen from this class, what is the probability that
at least one of these 2 got an A? Answered by Paul Betts. 





n1/(n+1)! + n+1/n! 
20070318 

From Cody: How do you go about simplifying something like this;
n1/(n+1)! + n+1/n!? Answered by Steve La Rocque and Claude Tardif. 





How to find the odds for a lottery jackpot 
20060412 

From Harvey: Is it best to use factorials to calculate the odds of winning a lottery, such as the MegaMillions that is popular in the US, or is there a better way? Answered by Stephen La Rocque. 





Prove that p^n >= (p!)/(pn)! 
20060202 

From Rhydian:
PROVE:
p^{n} >= (p!)/(pn)!
Answered by Penny Nom. 





Four people are in a race 
20060126 

From Tammy: If 4 people are in a race, how many different placements, i.e., 1st, 2nd, 3rd, 4th, can there be and what is the equation? Answered by Penny Nom. 





112! 
20040528 

From Beatriz: 1) A bus driver collects identical sets of 5 coins from each passenger. If the totoal colledted was $21.83, how many pennies did the driver get??
2) How many terminal zero in the base 10 expression of 112! (factional) N! means N(N1)(N2) .....(2) (1). Answered by Penny Nom. 





n! > n^2 
20040330 

From Jose: How can you prove by mathematical induction that:
n! > n2. Answered by Penny Nom. 





When is 1! + 2! + 3! + ... + x! a square? 
20020819 

From Sarathy: Solve : 1! + 2! + 3! + ... + x! = y^{ 2} How do i find the solutions ? Answered by Claude tardif. 





Why is 0! = 1? 
20010130 

From Diane: Every math book always claims that 1!=1 and 0!=1 are givens, and that we should just memorize it. i understand the 1! part, but where is the basis for claiming that 0!=1???? Answered by Walter Whiteley. 





Four crayons 
20010110 

From Neyra Espinoza: You have four crayons (red, blue, yellow, green). If you line them up, how many different combinations can you get? Answered by Patrick Maidorn. 





n! = 42(n2)! 
20000721 

From Damon Bailey: Solve n! = 42(n2)! Answered by Paul Betts. 





10,000! 
20000721 

From Lauren: Hi I was just wondering if you could tell me how many zeros are in 100,000! (factorial.) Answered by Denis Hanson. 





Six digit numbers using 1,2,5,6,7, and 9 
20000320 

From Rachel: How many different sixdigit numbers can you make using the digits 1,2,5,6,7, and 9? How many of these six digit numbers are divisible by six? Answered by Claude Tardif and Denis Hanson. 





0! + 1! + 2! + 3! + ... + 2000! 
19991021 

From Melissa: My name is melissa. I am a 9th grade student I am having trouble finding out how to do this: What is the tens digit of 0! + 1! + 2! + 3! + ... + 2000! I know how to find these, but my calculator cant go any higher than 69! Is there any way i can do this problem? Answered by Penny Nom. 





0! 
19970302 

From Donna D. Hall: I am looking for a quick and easy explanation as to why 0! is 1. Answered by Walter Whiteley and Denis Hanson. 





Divisibility of 2n choose n. 
19960924 

From Kathy Doan: Can you prove that "2n choose n" is not divisible by 3, 5, and 7 for infinitely many n? Answered by Penny Nom. 

