Modular arithmetic - is this a "legal" substitution?

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I know that

$$a \equiv b ~(\text{mod}~3)$$ and $$c \cdot a \equiv 1 ~(\text{mod}~3)$$

Can I substitute $a$ with $b$? I mean:

$$c\cdot b \equiv 1 ~(\text{mod}~3)$$

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Yes!

We have $b = a + 3k$, so $$c\cdot b = c\cdot(a + 3k) = c\cdot a + c\cdot 3k \equiv c\cdot a (\mod 3)$$