Describe a real life multiplication situation for which an estimate make sense

So, let's say that I have 3 bills: a 50 dollar bill, a 100 dollar bill and 1 dollar bill.

I need to buy 9 pens for my friends, and each of the pens costs 4.34 dollars.

I want to know if I can just take the 50 dollar bill, since I'm not too comfortable to carry the 100 dollar bill the whole day.

A quick estiate tells me that 10 times 5 is 50, and since both numbers are less, then in fact, 50 dollar bill will be enough!