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afla necunoscuta:
a) ( 13 - 3 x 4 ) x 18 : 6 = 6 - a : 9
b) ( a + 6 x 2 + 9 x 5 ) - 2 x ( 17 - 7 ) = 2 x 5 + 6 x 5
c) 5 x 7 + a x b = 75
d ) 8 x a + 9 x b = 67