Difference between revisions of "2019 Mock AMC 10B Problems/Problem 22"
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==Solution== | ==Solution== | ||
− | + | <math>15^n - 7^n \equiv 7^n - 7^n \equiv 0</math> <math>\text{mod}</math> <math>8</math> for all integer <math>n</math>. Therefore, <math>S = \{0, 8, 16, 24,...,248\}</math>. Since the sum of the elements in <math>S</math> is <math>8 + 16 + 24 + ... + 248 = 8(1 + 2 + 3 + ... + 31) = 8 \cdot \frac{31(31 + 1)}{2} = 2^7 \cdot 31</math>, the answer is <math>2 + 7 + 31 = \boxed{\text{(A)} 40}</math>. |
Revision as of 20:25, 3 November 2019
Problem
Let be the set of all possible remainders when
is divided by
, where
is a positive integer and
is the number of elements in
. The sum
can be expressed as
where
are positive integers and
and
are as small as possible. Find
.
Solution
for all integer
. Therefore,
. Since the sum of the elements in
is
, the answer is
.