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The positive integer n is greater than 10. What is the remainder when the positive integer n is divided by 12?
(1) n is 20 more than a multiple of 36.
(2) n is 4 less than a multiple of 3 and n is 4 less than a multiple of 8.
Target question: What is the remainder when the positive integer n is divided by 12? Statement 1: n is 20 more than a multiple of 36. In other words, n = 36k + 20 for some integer k
Let's do some rewriting.
n = 36k + 20
=
(12)(3)k + 12 + 8
=
12(3k + 1) + 8
=
12(some integer) + 8
Here, we can see that n equals 8 more than some multiple of 12
This means that,
if we divide n by 12, the remainder will be 8 Since we can answer the
target question with certainty, statement 1 is SUFFICIENT
Statement 2: n is 4 less than a multiple of 3 and n is 4 less than a multiple of 8 Let's examine both pieces separately:
n is 4 less than a multiple of 3In other words, n = 3k - 4 for some integer k
If we add 4 to both sides, we get n + 4 = 3k
This means that
n+4 is a multiple of 3n is 4 less than a multiple of 8In other words, n = 8j - 4 for some integer j
If we add 4 to both sides, we get n + 4 = 8j
This means that
n+4 is a multiple of 8If n+4 is a multiple of 3 AND 8, then we can conclude that n+4 is a multiple of 24
ALSO, if n+4 is a multiple of 24, then we can conclude that n+4 is a multiple of 12
Finally, if if n+4 is a multiple of 12, we can write: n+4 = 12q for some integer q
We can also write: n+4 = 12q - 12 + 12
[you'll see why shortly]n+4 = 12(q - 1) + 12
n =
12(q - 1) + 8
[I subtracted 4 from both sides]n =
12(some integer) + 8
Once again, we can see that n equals 8 more than some multiple of 12
This means that,
if we divide n by 12, the remainder will be 8 Since we can answer the
target question with certainty, statement 2 is SUFFICIENT
Answer:
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