Maths Olympiad Prep

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Number theory Difficulty 6.1 National Olympiad Prove it JBMO

Problem:
Let A=1472014A = 1 \cdot 4 \cdot 7 \cdot \ldots \cdot 2014 be the product of the numbers less or equal to 20142014 that give remainder 11 when divided by 33. Find the last non-zero digit of AA.

Solution

Solution:
Grouping the elements of the product by ten we get:
(30k+1)(30k+4)(30k+7)(30k+10)(30k+13)(30k+16)(30k+19)(30k+22)(30k+25)(30k+28)==(30k+1)(15k+2)(30k+7)(120k+40)(30k+13)(15k+8)(30k+19)(15k+11)(120k+100)(15k+14) \begin{aligned} & (30k+1)(30k+4)(30k+7)(30k+10)(30k+13)(30k+16) \\ & (30k+19)(30k+22)(30k+25)(30k+28) = \\ & = (30k+1)(15k+2)(30k+7)(120k+40)(30k+13)(15k+8) \\ & (30k+19)(15k+11)(120k+100)(15k+14) \end{aligned}
(We divide all even numbers not divisible by five, by two and multiply all numbers divisible by five with four.)
We denote Pk=(30k+1)(15k+2)(30k+7)(30k+13)(15k+8)(30k+19)(15k+11)(15k+14)P_k = (30k+1)(15k+2)(30k+7)(30k+13)(15k+8)(30k+19)(15k+11)(15k+14). For all the numbers not divisible by five, only the last digit affects the solution, since the power of two in the numbers divisible by five is greater than the power of five. Considering this, for even kk, PkP_k ends with the same digit as 127389141 \cdot 2 \cdot 7 \cdot 3 \cdot 8 \cdot 9 \cdot 1 \cdot 4, i.e. six and for odd kk, PkP_k ends with the same digit as 177339691 \cdot 7 \cdot 7 \cdot 3 \cdot 3 \cdot 9 \cdot 6 \cdot 9, i.e. six. Thus P0P1P66P_0 P_1 \ldots P_{66} ends with six. If we remove one zero from the end of all numbers divisible with five, we get that the last nonzero digit of the given product is the same as the one from 620112014410167968026 \cdot 2011 \cdot 2014 \cdot 4 \cdot 10 \cdot 16 \cdot \ldots \cdot 796 \cdot 802. Considering that 46284 \cdot 6 \cdot 2 \cdot 8 ends with four and removing one zero from every fifth number we get that the last nonzero digit is the same as in 44267847968021476794 \cdot 4^{26} \cdot 784 \cdot 796 \cdot 802 \cdot 1 \cdot 4 \cdot \ldots \cdot 76 \cdot 79. Repeating the process we did for the starting sequence we conclude that the last nonzero number will be the same as in 266401001602202806132677338792 \cdot 6 \cdot 6 \cdot 40 \cdot 100 \cdot 160 \cdot 220 \cdot 280 \cdot 61 \cdot 32 \cdot 67 \cdot 73 \cdot 38 \cdot 79, which is two.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.