SOLUTION: A list of whole numbers from 1 to 2015 is written on a sheet of paper. All the multiples of 5 are then struck off from the list. What is the last digit of the product of the remain

Algebra.Com
Question 1162771: A list of whole numbers from 1 to 2015 is written on a sheet of paper. All the multiples of 5 are then struck off from the list. What is the last digit of the product of the remaining numbers?
Answer by ikleyn(52925)   (Show Source): You can put this solution on YOUR website!
.

To get the answer, we should look and trace the last digit, only.


Regarding last digits, for our first numbers 1, 2, 3, 4, , 6, 7, 8, 9,  the last digit of the product is


    1*2*3*4*6*7*8*9 = 6 (mod10)   <<<---===  I wrote the LAST digit only (!!!)


Now, these factors will be cyclically repeated for every 10 numbers (with excluded 5 and 0).


Therefore, the last digit of the product, created by every 10 numbers, is 6.


    6*6 = 36 = 6 (mod10),


therefore, the last digit of the product of all these factors from 1 to 2010 (with excluded multiples of 5) is 6.


Now, to answer the final question, we should calculate

  
    6 * 11*12*13*14.


or, tracing the last digit only,  the product


    6 * 1*2*3*4 = 4  (mod10).


ANSWER.  The last digit of the number under the question is 4.

Solved.



RELATED QUESTIONS

a list of whole number from 1 to 2015 is written on a sheet of paper. All multiples of 5... (answered by ikleyn)
a list of whole numbers from 1 to 50 is written on a piece of paper. all the multiples of (answered by Edwin McCravy,ikleyn)
Each of the numbers 1-10 is written on a sheet of paper and the ten sheets of paper are... (answered by ewatrrr)
Each of the numbers 5 - 15 is written on a sheet of paper. The eleven sheets of paper are (answered by mccravyedwin)
The numbers 1 to 13 are written on 13 separate slips of paper and placed in a box. A... (answered by greenestamps)
two equal maximum sized circular plates are cut off from a circular paper sheet of... (answered by josgarithmetic)
The numbers 1 to 10 are written on sheets of paper and placed in a hat. If one sheet of... (answered by stanbon)
the numbers 1 through 50 are written with red markers on slips of paper while the numbers (answered by Boreal)
Given the sequence 3;6;9;....;60. a)Determine the sum of all the natural numbers from... (answered by math_helper)