What is the last two digits of: 1 1 2 0 1 8 ?
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Let N = 1 1 2 0 1 8 . We need to find N m o d 1 0 0 .
N ≡ 1 1 2 0 1 8 (mod 100) ≡ ( 1 0 + 1 ) 2 0 1 8 (mod 100) ≡ ( 1 0 2 0 1 8 + 2 0 1 8 × 1 0 2 0 1 7 + ⋯ + 2 2 0 1 8 × 2 0 1 7 × 1 0 2 + 2 0 1 8 × 1 0 + 1 ) (mod 100) ≡ 8 1 (mod 100)
Notice that:
1 1 2 m o d 1 0 0 = 2 1 ,
1 1 3 m o d 1 0 0 = 3 1 ,
1 1 4 m o d 1 0 0 = 4 1 … etc
⟹ 1 1 2 0 1 8 = 1 1 2 0 1 0 + 8 m o d 1 0 0 = ( 1 ) ( 8 1 ) = 8 1 m o d 1 0 0 ⟹ the last two digits are 81.
1 1 2 0 1 8 = 1 8 when we look for the last digit, so last digit must be 1 .
Then we only need to look for the last-second digits.
It is 8 .
So, it is 8 1 .
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Pascal’s triangle has a property where if all the digits in the n th row are read, it gives the n th power of 11. ( 2 0 1 8 2 0 1 8 ) = 1 , giving the last digit. ( 2 0 1 7 2 0 1 8 ) = ( 1 2 0 1 8 ) = 2 0 1 8 , so the last digits are ...20181. This leaves 81 as the last two digits.