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# Help ASAP

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In the statement below, the two blanks can be filled by positive single-digit numbers in such a way that the statement is always true:
$$\text{If }2x\equiv y+5\ (\bmod\ 9)\text{, then }x\equiv \underline{\ \ \ }\,y+\underline{\ \ \ }\ (\bmod\ 9).$$

What is the product of the two digits that go in the blanks?

May 17, 2018

#1
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Hey RB!

I'm not that good at number theory but here is what I have:

Multiplying both sides of the congruence:

$$2x\equiv y+5\pmod 9$$

by 5 gives:

$$10x \equiv 5y+25\pmod 9,$$

then reducing both sides modulo 9 gives

$$x\equiv 5y+7\pmod 9.$$

Thus, the product of the blanks is $$5\cdot 7=\boxed{35}.$$

I hope this helped,

Gavin

May 17, 2018