Let x represent the number
.. 1 +3x . . . . . . one more than three times a number
.. x -5 . . . . . . . the number decreased by 5
.. 1 +3x < x -5 . . . . the relation specified by the problem statement
.. 2x < -6 . . . . . . . subtract x+1
.. x < -3 . . . . . . . . . divide by the coefficient of x. (Positive, so < stays the same.)