2006 AIME I Problems/Problem 4
Problem
Let be the number of consecutive
's at the right end of the decimal representation of the product
Find the remainder when
is divided by
.
Solution
A number in decimal notation ends in a zero for each power of ten which divides it. Thus, we need to count both the number of 5s and the number of 2s dividing into our given expression. Since there are clearly more 2s than 5s, it is sufficient to count the number of 5s.
One way to do this is as follows: of the numbers
have a factor of
.
have a factor of
.
have a factor of
. And so on. This gives us an initial count of
. Summing this arithmetic series of
terms, we get
. However, we have neglected some powers of
- every
term for
has an additional power of
dividing it, for
extra; every n! for
has one more in addition to that, for a total of
extra; and similarly there are
extra from those larger than
and
extra from
. Thus, our final total is
, and the answer is
.
See also
2006 AIME I (Problems • Answer Key • Resources) | ||
Preceded by Problem 3 |
Followed by Problem 5 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |