It can happen from time to time that your company's sales forecasts are a bit off. If this has happened to you in your career before, be sure to use that as an example. The hiring manager is looking for a reply that will show them that you have the tenacity to try rather than just deem the target impossible and give up as a result.
"I have received very lofty sales targets before. Rather than treating the target as impossible, I will break it down a bit and tackle parts of it at a time. I like the challenge so would definitely treat the target as attainable unless proven otherwise."