DEPARTMENT: PUZZLE TIME SOLUTIONS

WRITE IT AS


      Recall that a composite number is a nonprime positive integer. We will call a composite number n a simple composite if it is the product of distinct primes and a complex composite if it has a factor of the form p2 for some prime p.
       Write 12,167 in the form a1 + a2 +
××× + a9 + b where each ai is a complex composite and b is a simple composite.

Weng Shao

Observe that (x + y + z)3 = x3 + 3x2y + 3x2z + 3xy2 + 6xyz + 3xz2 + y3 + 3y2z + 3yz2 + z3.
So
12,617 = (23)3 = (5 + 7+11)3 = 53 + 3
×52×7 + 3×52 ×11 + 3 ×5 ×72 + 3 ×5 ×112 + 73 + 3× 72 ×11 + 3 ×7×112 + 113 + 3×2× 5× 7× 11


Nathan Collins

a1 = a2 =  ··· = a9 = 22  and b = 7 · 1733

36 + 12131 = 12167

Josiah Thornton 

233 = (21 + 2)3 = 213 + 3
× 212 ×2 + 3 × 21× 22 + 23
21 3 + 2906 = (19 + 2)3 + 2906 =

193 + (3× 192 ×2 + 3 ×19× 22 + 23 ) + 2906 = 173 + 3× 172 ×2 + 3 ×17× 22 + 23 + (3× 192 ×2 + 3 ×19× 22 + 23 )
+ 2906 =  12167
a1 = 173, a2 = a3 = 3
×19× 2, a4 = a5 =  3× 172, a6 =  3 ×19× 22 , a7 = 3 ×17× 22, a8 = a9 = 23
b = 2906 = 2
× 1453

Travis Willse

      The sum of the digits of 12167 is 17 = 8 (mod 9), so the largest multiples of 9 smaller than 12167 are 12159, 12150, 12141, 12132... The last two digits of 12132 are 32, a multiple of 4, so 12132 is a multiple of 4, and so is (12132)/9 = 1348. Set ai = 1348 for i = 1, . . . , 9 (22 | 1348, so each ai is a "complex composite") and b = 12167-12132 = 35 = 5 · 7 (b thus a simple composite). Then, we have 12167 written in the required form:
12167 = 1348 + · · · + 1348 + 35
       By adding and subtracting multiples of 4 from the ai, we can write 12167 in the required form with distinct ai, too:
12167 = 1332 + 1336 + · · · + 1360 + 1364 + 35

Next Page

10