There is no need to do any calculation. You simply need the rate of the mules to be able to answer the question. Statement 2 is your answer. If you want some more information on this take a look here: GMAT Work

From the prompt, we know that 16 horses can do the job in 24 minutes. This means ONE horse does (1/16) of the job in 24 minutes, so ONE horse would take 16*24 = 384 minutes to do the job. Hence each horse has a rate of (1/384) of the job per minute.

If 12 horses work for 14 minutes, we have 12*14 = 168 horse-minutes of work. That means we've accomplished 168 * (1/384), or (7/16) of the job.

The next issue is whether the prompt means 15 additional minutes or 15 minutes total. If it means 15 minutes total, then S1 and S2 are BOTH sufficient, as the mules will not be able to do enough work in 1 minute to finish the job.

If it means 15 additional minutes, then we need the mules' rate. S1 doesn't give us that, but S2 does, so only S2 is sufficient.