I can almost guarantee you use it regularly, just dont realize it. You see something 3 for a dollar. You want one. You figure out the price of one (x). You just solved for x
Average subscriber (x) pays $25. Goal is $500. How many subscribers do I need?
$500=x*$25
Now, calculus, on the other hand, is just what they stupidly teach kids who should be learning stats.