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a=b(mod n)
if their difference a − b is an integer multiple of n. The number n is called the modulus of the congruence.
so:
7x=8(mod9); 7x - 8 has to be a multiple of 9, trying iterative values:
7(1) - 8 = -1 no
7(2) - 8 = 6 no
7(3) - 8 = 13 no
7(4) - 8 = 20 no
7(5) - 8 = 27 yes
7*5 = 8(mod9)
