How to do triangulation in this case?

I think that few people would know whether this number is high or low. One way to put it into perspective would be to estimate how much each segment spends on gum per year and then cross-check with your personal experience.

However, if you want to ace the case it could be extended quite a bit compared to how it is written. Notable, you could segment the users further - one segment that comes to mind are smokers trying to quit. These often chew gum as a substitute, and probably go through a pack a day. Similar usage patterns could be identified for people taking gum for medical reasons, such as halitosis.

And this then raises questions related to the price, here assumed as 80 cents. Nicotine gum would certainly be higher, and there even are premium brands such as "5". This would let you segment further by assuming different average gum prices for different user segments.

Hope this helps - there's definitely a lot more that can be done with this case, and the obvious approach is to segment more fine-grained. And the more fine-grained you are, the more you can then compare it to your own experience and the experiences of those around you...