Just For Fun

Algebra Level 2

201 6 2 + 201 6 3 \large 2016^2 + 2016^3

Which digit does not appear when this integer is written out?

Feel free to use a calculator for this one.

7 6 They're all there! 2 3

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3 solutions

Eli Ross Staff
Feb 19, 2016

Just for some (silly) fun, we can actually solve this pretty easy without multiplying out the answer at all, if we assume that the correct answer is listed.

Note that our number is 201 6 2 ( 1 + 2016 ) = 201 6 2 2017. 2016^2 \cdot (1+ 2016) = 2016^2 \cdot 2017. The number of digits it has is log 10 201 6 2 2017 . \lceil \log_{10} 2016^2 \cdot 2017 \rceil. Note that 201 6 2 2017 = 2.01 6 2 × 2.107 × 1 0 9 8 × 1 0 9 , 2016^2 \cdot 2017 = 2.016^2 \times 2.107 \times 10^9 \approx 8 \times 10^9, so it has 9 + 1 = 10 9+1=10 digits.

Let's suppose 1 digit A A was missing, so one other digit B B is there twice. The sum of the digits would then be ( 0 + 1 + 2 + + 9 ) + B A = 45 + B A . (0+1+2+\ldots+9) + B - A = 45 + B - A. The number has to be divisible by 9 , 9, so B A B-A must be a multiple of 9. This is only possible if { A , B } = { 0 , 9 } . \{A,B\} = \{0,9\}. But neither of these digits are choices. So, no digits are missing.

Ric Best
Jan 9, 2016

If you put the numbers in your calculator you can see which digits are there. They all are.

Josiah Kiok
Feb 19, 2016

If there is no room on your calculator, put this in wolfram alpha or spotlight.(2016^2+2016^3)

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