Cubes - mind gobblers

How many three-digit numbers exist such that their cubes end in 44?

18 27 20 16

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1 solution

Daniel Branscombe
Aug 19, 2014

let the number be given by

n = 100 x + y n=100*x+y

with 1 < = x < = 9 1<=x<=9 and 0 < = y < = 99 0<=y<=99

then

n 3 = ( 1 0 2 x + y ) 3 n^3=(10^2*x+y)^3

n 3 = 1 0 6 x 3 + 3 1 0 4 x 2 y + 3 1 0 2 x y 2 + y 3 n^3=10^6*x^3+3*10^4*x^2y+3*10^2*x*y^2+y^3

now to get the last two digits we simply take this m o d 100 mod 100 which simplifies down to

n 3 = y 3 n^3 = y^3 m o d mod 100 100 now last digit, d d , needs to satisfy d 3 = 4 d^3 = 4 m o d mod 10 10 , and this is only true for d = 4 d=4 . So now there are only 10 possible values for y, namely ( 4 , 14 , 24 , 34 , 44 , 54 , 64 , 74 , 84 , 94 ) (4,14,24,34,44,54,64,74,84,94) trying each of these we get that only

1 4 3 = 44 14^3 = 44 m o d mod 100 100 and 6 4 3 = 44 64^3 = 44 m o d mod 100 100 .

Thus there are 2 possible values for y and 9 possible values of x giving 18 \boxed{18} total.

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