Show the code you've written to generate random number 1 to 5, inclusive, the insertNumbers() method. The code you showed originally was not correct.
Use your brain, not a video: To define the point at which the tremor begins, a random number is generated 0 to 19, exclusive. A for() loop could be used to 'visit' the 3 elements to the left of the index (less than) defined by the random number, the element at the index, and the 3 elements to the right of the index (greater than). That's just simple math, startIndex = index - 3, endIndex = index + 3. Care must be taken to ensure that startIndex >= 0 and endIndex <= 19. The whole thing: a couple simple equations, a couple if() statements, and a for() loop. Easy stuff.