1985 AHSME Problems/Problem 9

Problem

The odd positive integers $1, 3, 5, 7, \ldots$ , are arranged into five columns continuing with the pattern shown on the right. Counting from the left, the column in which $1985$ appears is the

$[asy] int i,j; for(i=0; i<4; i=i+1) { label(string(16*i+1), (2*1,-2*i)); label(string(16*i+3), (2*2,-2*i)); label(string(16*i+5), (2*3,-2*i)); label(string(16*i+7), (2*4,-2*i)); } for(i=0; i<3; i=i+1) { for(j=0; j<4; j=j+1) { label(string(16*i+15-2*j), (2*j,-2*i-1)); }} dot((0,-7)^^(0,-9)^^(2*4,-8)^^(2*4,-10)); for(i=-10; i<-6; i=i+1) { for(j=1; j<4; j=j+1) { dot((2*j,i)); }}[/asy]$

$\mathrm{(A)\ } \text{first} \qquad \mathrm{(B) \ }\text{second} \qquad \mathrm{(C) \ } \text{third} \qquad \mathrm{(D) \ } \text{fourth} \qquad \mathrm{(E) \ }\text{fifth}$

Solution

Considering each integer modulo $16$ gives the following pattern: $[asy] int i,j; for(i=0; i<4; i=i+1) { label(string(2*i+1), (2*i,-2*1)); label(string(15-2*i), (2*(i-1),-2*1.35)); label(string(2*i+1), (2*i,-2*1.7)); label(string(15-2*i), (2*(i-1),-2*2.05)); } [/asy]$

We therefore observe that all numbers congruent to $1 \pmod{16}$ will appear in the second column, and since $1985 \equiv 1 \pmod{16}$ , the answer is $\boxed{\text{(B)} \ \text{second}}$ .

1985 AHSME (Problems • Answer Key • Resources)
Preceded by Problem 8	Followed by Problem 10
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1985 AHSME Problems/Problem 9

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