I assume the problem reads "he uses 1/6 bottle of glue to make 1 robot."
If 1 robot requires 1/6 bottle of glue, then how many robots can Jack make with 1 bottle of glue?
6 * 1/6 = 1 bottle of glue, so Jack can make 6 robots with 1 bottle of glue
With 4 bottles of glue, Jack can make 6 * 4 = 24 bottles of glue
If x = the number of robots Jack can make per 1/6 bottle of glue, then the equation would be:
1/6 * x = 4