Both of the responses you have received use some kind of formal algebra to solve the problem, as do all the examples in the lesson referenced by the third tutor.
Those methods are fine, and they might need to be use in more complicated problems. But this kind of basic problem can be solved in very little time informally, using logical reasoning.
In this example, we are told that 4 men can do a job in 8 days; then we are asked to find how many days it would take 16 men to do the job.
Common sense tells us that increasing the number of workers by a factor of 4 will decrease the required amount of time by a factor of 4.