How many positive integers n have the property that the remainder of dividing 2003 by n is equal to 23?
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2 0 0 3 ≡ 2 3 ( m o d n ) ⟹ 2 0 0 3 − 2 3 ≡ 0 ( m o d n ) ⟹ 1 9 8 0 ≡ 0 ( m o d n )
Thus n should be a divisor of 1 9 8 0 . Also note that n > 2 3 to give a remainder of 2 3 when it divides 2 0 0 3 .
Prime factorization of 1 9 8 0 gives 1 9 8 0 = 2 2 × 3 2 × 5 × 1 1 . Thus 1 9 8 0 has 3 6 divisors. Listing out the divisors, we find that there are 1 4 divisors which are less than 2 3 , so our required answer is 3 6 − 1 4 = 2 2 .
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If 2003 is divided by n, the result would be a whole number 'N' plus a remainder.
If the remainder is 23, then,
n 2 0 0 3 = N + n 2 3
or, N = n 1 9 8 0
or, n = N 1 9 8 0
The factors of 1980,
1, 2, 3, 4, 5, 6, 9, 10, 11, 12, 15, 18, 20, 22, 30, 33, 36, 44, 45, 55, 60, 66, 90, 99, 110, 132, 165, 180, 198, 220, 330, 396, 495, 660, 990 and 1980
n must be greater than 23 [ otherwise the remainder cannot be 23 ]
Therefore, there are 22 possible values of n = 30, 33, 36, 44, 45, 55, 60, 66, 90, 99, 110, 132, 165, 180, 198, 220, 330, 396, 495, 660, 990 and 1980
Answer : 22