How many permutations (x1; x2; x3; x4) of the set of integers {1; 2; 3; 4} have the property that the sum x1.x2 + x2.x3 + x3.x4 + x4.x1 is divisible by 3?

kakak ada yg ngerti maksudnya gimana? tolong dibantu stepsnya donk. katanya ga perlu kalkulator... terima kasih yg bantuin ane cium dhe