How many three-digit numbers exist such that their cubes end in 44?
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let the number be given by
n = 1 0 0 ∗ x + y
with 1 < = x < = 9 and 0 < = y < = 9 9
then
n 3 = ( 1 0 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
now to get the last two digits we simply take this m o d 1 0 0 which simplifies down to
n 3 = y 3 m o d 1 0 0 now last digit, d , needs to satisfy d 3 = 4 m o d 1 0 , and this is only true for d = 4 . So now there are only 10 possible values for y, namely ( 4 , 1 4 , 2 4 , 3 4 , 4 4 , 5 4 , 6 4 , 7 4 , 8 4 , 9 4 ) trying each of these we get that only
1 4 3 = 4 4 m o d 1 0 0 and 6 4 3 = 4 4 m o d 1 0 0 .
Thus there are 2 possible values for y and 9 possible values of x giving 1 8 total.