How many bottles of shampoo are produced in the world a year?
Not enough, apparently.
Let's start with the US market: 300X10^6 people. About 60% use shampoo (others are bald or use soap). They go through about one bottle every two months, so that is about 1.8X10^9. Human population is about 6.6 X 10^9. As far as the undeveloped nations, most people don't use shampoo. The developed world is about four times the size of the US population or approximately 1.2 X 10^9. People in the US wash their hair more than in Europe, but I will neglect this. My estimate is about 5X10^9 bottles.
Enough to wash everyone's hair.
none where difficult. I was asked to design an elevator. next was to create an algorithm to predict how many sheets are needed to print a certain number of graphics.